LECTURE III. PHYSIOLOGICAL EFFECTS OF RADIATION. Visibility. 20. The most important physiological effect is the visibility of the narrow range of radiation, of less than one octave, between wave length 76 X 10~6 and 39 X 1Q-6. The range of intensity of illumination, over which the eye can see with practically equal comfort, is enormous: the average intensity of illumination at noon of a sunny day is nearly one million times greater than the illumination given by the full moon, and still we can see fairly well in either case; that is, the human eye can adapt itself to enormous differences in the intensity of illumination, and that so perfectly that it is difficult to realize the differences in intensity without measuring them. The photo- graphic camera realizes it. An exposure taken in T^ second with TV opening of the diaphragm in full sunlight usually gives a better photograph than an exposure of 10 minutes at full opening, in the light of the full moon. The ratio of time of exposure in the two cases, however, is about 1 to 1,000,000, thus showing the difference in the intensity of illumination. Also, the disk of the moon, when seen in daylight, has about the same intensity as the sky — somewhat more than the cloudless sky, less than white reflecting clouds. As the surface of the moon's disk, of one-half degree diameter, is about TffsWtf the surface of the sky, it thus follows that the daylight reflected from the sky is about 100,000 times more intense than the light of the full moon. The organ by which we perceive the radiation, the human eye (Fig. 20), contains all the elements of a modern photographic camera — an achromatic lense: the lense L, of high refractive power, enclosed between the two transparent liquids A and B which correct the color dispersion, that is, give the achromatic property; a diaphragm: the iris 7, which allows the increase or decrease of the opening P, the pupil; a shutter: the eyelids and 87 38 RADIATION, LIGHT, AND ILLUMINATION the sensitive plate or retina R. The nerves of vision end at the back of the retina, and in the center of the retina is a spot F, the "sensitive spot " or " fova," at which the retina is very thin, and the nerve ends specially plentiful. At this spot we thus see sharpest and clearest, and it is this spot we use for seeing by turning the eye so as to fix on it the image of the subject we desire to see, while the image on the rest of the retina is used merely for orientation. The adaptability to the enormous FIG. 20. range of intensity of illumination, which as seen we meet in nature, is secured: (1). By changing the opening and thereby the amount of light admitted to the eye, by contracting or opening the pupil P. This action is automatic. In low intensity of illumination the pupil thus is wide open and contracts at higher intensities. As this automatic action takes an appreciable, though short time, a flash light photograph shows the pupil of the eye fully open and thereby gives a staring impression to the faces which is avoided by keep- ing a photographically inactive light, as a candle, burning outside of the field of the camera when preparing for a flash light photo- graph. (2). By the fatigue of the optic nerves, exposed to high inten- sity of illumination, the nerves becomes less sensitive, while at low intensity they rest and thus become more sensitive, and the differences of sensation are hereby made very much less than corresponds to the differences of intensity of radiation. There- fore, when entering a brightly illuminated room from the dark- ness we are blinded in the first moment, until the eye gets accustomed to the light, that is, the nerves become fatigued and so reduce the sensation of light. Inversely, when stepping from a bright room into the darkness we first see almost nothing until the eye gets accustomed to the darkness, that is, the nerves of vision are rested and their sensitivity thus increased so as to per- ceive the much lower intensity of illumination. (3). By the logarithmic law of sensation. The impression made on our senses, eye, ear, etc., that is, the sensation, is not propor- tional to the energy which produces the sensation, that is, the PHYSIOLOGICAL EFFECTS OF RADIATION. 39 intensity of the light, the sound, etc., but is approximately proportional to its logarithm and the sensation, therefore, changes very much less than the intensity of light, etc., which causes the sensation. Thus a change of intensity from 1 to 1000 is 1000 times as great a change of intensity as from 1 to 2, but the change of sensation in the first case, log 1000 = 3, is only about 10 times as great as the change in the latter case, log 2 - 0.301. This logarithmic law of sensation (Fechner's Law), while usu- ally not clearly formulated, is fully familiar to everybody, is con- tinuously used in life, and has been used from practical experience since by-gone ages. It means that the same relative or percent- age change in intensity of light, sound, etc., gives the same change of sensation, or in other words, doubling the intensity gives the same change in sensation, whether it is a change of intensity from one candle power to two candle power, or from 10 to 20, or from 1000 to 2000 candle power. It is obvious that the change of sensation is not proportional to the change of intensity; a change of intensity of light by one candle power gives a very marked change of sensation, if it is a change from one to two candle power, but is unnoticeable, if it is a change from 100 to 101 candle power. The change of sensation thus is not proportional to the absolute change of intensity — one candle power in either case — but to the relative or percentage change of intensity, and as this is 100 per cent in the first, 1 per cent in the latter case, the change of sensation is marked in the first, unnoticeable in the latter case. This law of sensation we continuously rely upon in practice. For instance, when designing an electrical distribution system for lighting, we consider that the variation of voltage by 1 per cent is permissible as it gives a change of candle power of about 5 per cent, and 5 per cent variation is not seriously noticeable to the eye. Now this 5 per cent change of candle power may be a change from 1 to 0.95, or by -fa candle power, or it may be a change from 1000 to 950, or by 50 candle power, and both changes we assume, and are justified herein from practical experience, to give the same change of sensation, that is, to be near the limits of permissi- bility. This law of sensation (Fechner's Law) means : If i = intensity of illumination, as physical quantity, that is, 40 RADIATION, LIGHT, AND ILLUMINATION. in meter-candles or in watts radiation of specified wave length, the physiological effect given thereby is : L = A log V %> where A is a proportionality constant (depending on the physio- logical measure of L) and \ is the minimum perceptible value of illumination or the "threshold value," below which sensation ceases. The minimum value of change of intensity i, which is still just perceptible to the average human eye, is 1.6 per cent. This, then, is the sensitivity limit of the human eye for changes of illumination. Obviously, when approaching the threshold value i0, the sensi- tivity of the eye for intensity changes decreases. The result of this law of sensation is that the physiological effect is not proportional to the physical effect, as exerted, for instance, on the photographic plate. The range of intensities permissible on the same photographic plate, therefore, is far more restricted. A variation of illumination within the field of vision of 1 to 1000, as between the ground and the sky, would not be seriously felt by the eye, that is, not give a very great difference in the sensation. On the photographic plate, the brighter portions would show 1000 times more effect than the darker portions and thus give bad halation while the latter are still under exposed. A photographic plate, therefore, requires much smaller variations of intensity in the field of vision than permissible to the eye. In the same man- ner the variations of intensity of the voice, used in speaking, are far beyond the range of impression which the phonograph cylin- der can record, and when speaking into the phonograph a more uniform intensity of the voice is required to produce the record, otherwise the lower portions of the speech are not recorded, while at the louder portions the recording point jumps and the voice breaks in the reproduction. 21. The sensitivity of the eye to radiation obviously changes with the frequency, as it is zero in the ultra-red, and in the ultra- violet — where the radiation is not visible — and thus gradually increases from zero at the red end of the spectrum to a maximum somewhere near the middle of the spectrum and then decreases again to zero at the violet end of the spectrum; that is, the physi- PHYSIOLOGICAL EFFECTS OF RADIATION. 41 ological effect produced by the same radiation power — as one watt of radiating power — is a maximum near the middle of the visible spectrum and decreases to zero at the two ends, about as illustrated by the curves in Fig. 21. Inversely, the mechanical equivalent of light, or the power required to produce the same physiological effect — as one candle power of light — is a maxi- mum near the middle of the spectrum and decreases from there to infinity at the end of the visible range, being infinite RED YELLOW GREEN BLUE VIOLET FIG. 21. in the ultra-red and ultra-violet, where no power of radiation can produce visibility. It thus varies about as indicated in Fig. 22. The mechanical power equivalent of light, thus, is not constant, as the mechanical energy equivalent of heat — which is 426 kgm. or 4.25 kile-joule per calorie — but is a function of the frequency, that is, of the color of radiation, with a maximum, probably not very far from 0.01 watt per candle power in the middle of the spectrum. When comparing, however, the physiological effects of different frequencies of radiation, that is, different colors of light, the diffi- culty arises that different colored lights cannot be compared photometrically, as all photometers are based on making the illu- mination produced by the two different sources of light equal, and when these sources of light are of different color they can never become equal. As long as the colors are not very different - two different shades of yellow or yellowish white and white — the eye can still approximately estimate the equality of intensity and 42 RADIATION, LIGHT, AND ILLUMINATION. thus compare them, though not as accurately as when the two sources of light are of the same color. With very great color differences, as green light and orange light, this is no longer feasible. However, an accurate comparison can still be made on the basis of equal ease in distinguishing objects. As the pur- FIG. 22. pose for which light is used is to distinguish objects, the correct comparison of lights obviously is on the basis of equal distinctness of objects illuminated by them; that is, two lights, regardless whether of the same or of different colors, give the same candle power, that is, the same physiological effect, if they enable us to distinguish objects with the same ease at the same distance. Experience has shown that the sharpest distinction, that is, the greatest accuracy in comparing different lights in this manner, is reached by determining the distance from the source of light at PHYSIOLOGICAL EFFECTS OF RADIATION. 43 which print of moderate size just ceases to be readable. For this purpose the print must be a mixture of letters which do not form intelligible words and the point which can be determined most accurately is where large letters, as capitals, are still readable, while small letters are already unreadable (see p. 174) . Obviously, in comparing different colors of light the object must be colorless, that is, the print be black on white. This method of comparison of the physiological effect, by what has been called the "lumino- meter," is theoretically the most correct, as it is independent of the color of light. It is, however, not as accurate as the compari- son by photometer, and thus the average of a number of observa- tions must be used. The only error which this method leaves is that due to the difference in the sensitivity of different eyes, that is, due to the differences between the sensitivity curves (Fig. 21), and this in most cases seems to be very small. 22. It is found, however, that the sensitivity curve for different colors of radiation is a function of the intensity of radiation; that is, the maximum sensitivity point of the eye is not at a definite frequency or wave length, but varies with the intensity of illumi- nation and shifts more towards the red end of the spectrum for high, towards the violet end of the spectrum for low intensity of illumination, and for illumination of very high intensity the maxi- mum physiological effect takes place in the yellow light, while for very low intensity of illumination it occurs in the bluish green light; that is, at high intensity yellow light requires less power for the same physiological effect than any other color of light, while for low intensity, bluish green light requires less power for the same physiological effect than any other color of light. Thus, if an orange yellow light, as a flame carbon arc, and a bluish green light, as a mercury lamp, appear of the same intensity from the distance of 100 feet, by going nearer to the lamps the orange yellow appears to increase more rapidly in intensity than the bluish green, and from a very short distance the former appears glaring bright, while the latter is disappointing by not showing anywhere near the same apparent intensity. Inversely, when going further and further away from the two lamps the orange yellow light seems to fade out more rapidly than the bluish green, and has practically disappeared while the bluish green is still markedly visible. A mercury lamp, therefore, can be seen from distances from which a much brighter yellow flame arc is practi- 44 RADIATION, LIGHT, AND ILLUMINATION. cally invisible, but inversely, from a very short distance the yellow light appears dazzling, while a mercury lamp of higher candle power appears less bright. Fig. 23 illustrates the change of sensitivity with intensity, by approximate curves of the variation of the relative sensitivity of the average human eye with the intensity i of illumination in v FIG. 23. meter candles (or rather log i) as abscissas, for red light, wave length 65.0; orange yellow light, wave length 59; bluish green light, wave length 50.5; and violet light, wave length 45.0. As seen for red light as well as violet light — the two ends of the visible spectrum — the sensitivity is low, while for orange yellow as well as bluish green light — near the middle of the visible range — the sensitivity is high. For bluish green light, however, the sensitivity is high at low and moderate intensities but falls off for high intensities, while for orange yellow light the sensitivity is high at high intensities and falls off at medium and low intensities and ultimately vanishes, that is, becomes invisible at intensities many times higher than those at which green light is still well visible. Red light vanishes from visibility still earlier than orange yel- low light, while violet light remains visible even at very low intensities. The vanishing points of the different colors of light, that is, PHYSIOLOGICAL EFFECTS OF RADIATION. 45 the minimum intensities which can just be perceived are, approxi- mately, at: Color red orange yellow green blue violet Wave length lw = 67 60.5 57.5 50.5 47 . 43 x 10~fl Meter-candles in- tensity .... i0 = 0.06 0.0056 0.0029 0.00017 0.00012 0.00012 Relative radiation power po = 10,000 1000 100 1 2 20 That is, the minimum visible amount of green light represents the least amount of power; the minimum visible amount of blue light requires twice as much power as green light; violet light 20 times as much, but yellow light 100 times and red light even 10,000 times as much power as green light at the threshold of visibility. While the intensity of radiation varies inversely proportional to the square of the distance, it follows herefrom that the physio- logical effect of radiation does not vary exactly with the square of the distance, but varies somewhat faster, that is, with a higher power of the distance for orange yellow or the long-wave end of the spectrum, and somewhat slower, that is, with a lesser power of the distance than the square, for bluish green or the short-wave end of the spectrum. This phenomenon is appreciable even when comparing the enclosed alternating carbon arc with the open direct current car- bon arc : by photometer, where a fairly high intensity of illumi- nation is used, the relative intensity of the two arcs is found somewhat different than by luminometer, that is, by reading distances nearer the lower limit of visibility. For low intensities, the alternating arc compares more favorably than for high intensities. It follows, therefore, that in the photometric comparison of illuminants, where appreciable color differences exist, the inten- sity of illumination at which the comparison is made must be given, as it influences the result, or the candle power and the distance of observation stated. 23. Not only the sensitivity maximum is different for low and for high intensity of illumination, but the shape of the sensi- tivity curve also is altered, and for low intensity is more peaked, that is, the sensitivity decreases more rapidly from a maximum towards the ends- of the spectrum than it does for high intensity 46 RADIATION, LIGHT, AND ILLUMINATION. of illumination as indicated by the curves in Fig. 24 which shows approximate sensitivity curves of the average human eye : (a) for every low illumination near the treshold value of visi- bility or 0.001 meter-candles; (b) for medium illumination, 4.6 meter-candles; (c) for very high illumination, 600 meter-candles. lu,*-. 1.65 I = 45.0 1.70 50.0 L t \ \ \ FIG. 24. (1 meter-candle is the illumination produced by 1 candle power of light intensity at 1 meter distance; N meter-candles, thus, the illumination produced by a light source of N candle power at 1 meter distance or of 1 candle power at— -= meter distance, etc.). VN As seen, curve (a) ends at wave length /„, = 61 X 10~fl; that is, for longer waves or orange and red light, 0.001 meter-candles is below the threshold value of visibility, hence is no longer visible. The maximum visibility, that is the sensitivity maximum of the human eye, lies at wave length. Z0 = 51.1, bluish green for very low intensity, curve (a). Z0 = 53.7, yellowish green for medium intensity, curve (b). 10 = 56.5, yellow for high intensity, curve (c). The sensitivity maximum varies with the intensity about as shown in Fig. 25; that is, it is constant in the bluish green for low intensities, changes at medium intensities in the range be- tween 0.5 and 50 meter-candles and again remains constant in the yellow for still higher intensities. PHYSIOLOGICAL EFFECTS OF RADIATION. 47 The sensitivity curves, as given in Fig. 24, have the general character of probability curves : where lwo is the wave length at maximum sensitivity and HQ is the sensitivity at this wave length, that is, the maximum sensi- tivity and ks is a constant which is approximately 120 for low, LOG t= - 3 I2 - 1 0 * 1 + a 4 3 57 - 0. X)l 0. bi 0 i 1 1 1 ^1 )0 100 ^x ME s ER-C, NDLE '/ ';\ V / / Sr, H=H< «•*• (j*~ i)2 ^s >c *~- , / [gg S FIG. 25. 62 for high intensities and changes in approximately the same range of intensities in which lwo changes; ks is also plotted in Fig. 25. This effect of the intensity of illumination on the sensitivity of the eye is very important in illuminating engineering as it deter- mines the color shades which are most effective for the particular purpose. For instance, in sending the light to great distances, for signalling, etc., the bluish green of the mercury lamp is best suited, carries farthest, and the yellow flame arc the poorest; the white carbon arc superior to the yellow flame arc, even where the latter is of greater intensity. Inversely, where a big glare of light is desired, as for decorative purposes, for advertising, etc., the yellow flame carbon arc is best suited, the bluish green mer- cury lamp disappointing. Apparent exceptions may exist : for instance, the long waves of the orange yellow penetrate fog better than the short waves of bluish green, and for lighthouses, where the important problem is to reach the greatest possible distance in fog, yellow light, thus, may be superior. In general, however, the bluish green is superior 48 RADIATION, LIGHT, AND ILLUMINATION. in visibility to the orange yellow for long distances, and inversely, the orange yellow is superior for short distances. At the limits of visibility the eye is very many times more sensitive to green light and, in general, high-frequency light, than to orange yellow and, in general, low-frequency light. A necessary result of the higher sensitivity of the eye for green light is the preponderance of green in gas and vapor spectra. As no special reason exists why spectrum lines should appear more frequently at one wave length than at any other and as the radia- tion is most visible in the green, this explains, somewhat, the tendency of most highly efficient illuminants towards a greenish or yellow color (as, for instance, the Welsbach mantel, the Nernst lamp, etc.). Pathological and Other Effects on the Eye. 24. Radiation is a form of energy, and thus, when intercepted and absorbed, disappears as radiation by conversion into another form of energy, usually heat. Thus the light which enters the eye is converted into heat, and if its power is considerable it may be harmful or even destructive, causing inflammation or burns. This harmful effect of excessive radiation is not incident to any particular frequency, but inherent in radiation as a form of energy. It is, therefore, greatest for the same physiological effect, that is, the same amount of visibility, for those frequencies of light which have the lowest visibility or highest power equiva- lent, that is, for the red and the violet and least for the green and the yellow, which for the same amount of visibility represent least power. Hence, green and greenish yellow light are the most harmless, the least irritating to the eye, as they represent the least power. We feel this effect and express it by speaking of the green light as "cold light" and of the red and orange light as "hot" or "warm." The harmful effect of working very much under artificial illumination is largely due to this energy effect, incident to the large amount of orange, red, and especially ultra- red in the radiation of the incandescent bodies used for illumi- nants and thus does not exist with "cold light," as the light of the mercury lamp. Blue and violet light, however, are just as energetic, or "hot," as orange and red light, and the reason that they are usually not recognized as such is that we have no means to produce efficiently PHYSIOLOGICAL EFFECTS OF RADIATION. 49 powerful blue and violet light, and if we could produce it would not be able to use it for illumination, due to the specific effects of this light which will be described in the following. If, in Fig. 26, the curve A represents roughly the mechanical power equivalent of light for average intensity, that is, the power required to produce the same physiological effect or the same candle power, the distribution of power in an incandescent lamp YELLOW GREEN FIG. 26. carbon filament would be somewhat like C. That is, the physio- logical effect falls off somewhat towards the green, as C drops more than A, and almost vanishes in the blue and violet, as C rapidly decreases, while A, the power required to give the same physiological effect, rapidly increases. From the yellow towards the red the physiological effect again decreases somewhat, but 50 RADIATION, LIGHT, AND ILLUMINATION. the radiation still increases towards the ultra-red. Dividing C by A then gives the distribution of the physiological effect, curve C', that is, of visibility, in the incandescent lamp spectrum, show- ing that the color of the light is yellow. Hg gives the distribu- tion of power in the mercury spectrum. It is shown in dotted lines, as the distribution is not continuous, but the power massed at definite points, the spectrum lines of mercury. Hg' then gives the visibility curve by dividing Hg by A. As seen, the ratio of the area of Hg* to Hg, that is, the ratio of the physiological effect to the power, is much less than the ratio of the area of C' to C; that is, the former produces for the same amount of visibility far less heat and thus is safer. 25. Excessive intensity, such as produced at a short-circuiting arc, is harmful to the eye. The human organism has by evolu- tion, by natural selection, developed a protective mechanism against the entrance of radiation of excessive power into the eye : at high intensity of illumination the pupil of the eye contracts and thus reduces the amount of light admitted, and a sudden exposure to excessive radiation causes the eyelids to close. This protective mechanism is automatic; it is, however, responsive mainly to long waves of radiation, to the red and the yellow light, but not to the short waves of green, blue and violet light. The reason for this is apparently that all sources of excessive radia- tion which are found in nature, the sun and the fire, are rich in red and yellow rays, but frequently poor in rays of short wave length, and, therefore, a response to short wave lengths alone would not be sufficient for protection as they might be absent in many intense radiations, while a response to long waves would be sufficient since these are always plentiful in the intense radiations found in nature. It is only of late years that illuminants, as the mercury lamp, which are deficient in the long waves, have been produced, and for these the protective action of the eye, by contracting the pupil, fails. This absence or reduction of the contraction of the pupil of the eye in the light of the mercury lamp is noticed when passing from a room well illuminated by incandescent lamps, to one equally well illuminated by mercury lamps and inversely. When changing from the incandescent light to the mercury light, the illumination given by the latter at first appears dull and inferior as the pupil is still contracted, but gradually gains in intensity as the pupil PHYSIOLOGICAL EFFECTS OF RADIATION. 51 opens; and inversely, coming from the mercury light to the incan- descent light, the latter first appears as a big glare of light, the pupil still being open, but gradually dulls down by the contraction of the pupil. This absence of the automatic protective action of the eye against light deficient in long waves is very important, as it means that exposure to excessive intensity of illumination by mercury light may be harmful, due to the power of the light, against which the eye fails to protect, while the same or even greater power of radiation in yellow light would be harmless, as the eye will pro- tect itself against it. The mercury lamp, therefore, is the safest illuminant, when of that moderate intensity required for good illumination, but becomes harmful when of excessive intensity, as when closely looking at the lamp for considerable time, when operating at excessive current. The possibility of a harmful effect is noticed by the light appearing as glaring. This phe- nomenon explains the contradictory statements occasionally made regarding the physiological effect of such illuminants. 26. Up to and including the green light, no specific effects, that is, effects besides those due to the power of radiation, seem yet to exist. They begin, however, at the wave length of blue light. I show you here a fairly intense blue violet light, that is, light containing only blue and violet radiation. It is derived from a vertical mercury lamp, which is surrounded by two concentric glass cylinders welded together at the bottom. The space be- tween the cylinders is filled with a fairly concentrated solution of potassium permanganate (strong copper nitrate solution or a cupric-ammon salt solution, though not quite so good may also be used) which is opaque to all but the blue and violet radiations. As you see, the light has a very weird and uncanny effect, is extremely irritating: you can see by it as the intensity of illumi- nation is fairly high, but you cannot distinguish everything, and especially the lamp is indefinite and hazy : you see it, but when you look at it it disappears, and thus your eye is constantly try- ing to look at it and still never succeeds, which produces an irritating restlessness. It can well be believed that long exposure to such illumination would result in insanity. The cause of this weird effect — which is difficult to describe — is that the sensitive spot on the retina, that is, the point on which we focus the image 52 RADIATION, LIGHT, AND ILLUMINATION. of the object which we desire to see, or the fova F in Fig. 19, is blue blind, that is, does not see the blue or violet light. Thus we see the lamp and other objects indistinctly on the outer range of the retina, but what we try to see distinctly disappears when focused on the blue blind spot F. This spot, therefore, is often called the " yellow spot," as we see yellow on it — due to the absence of the vision of blue at this particular place of the retina. To produce this effect requires the mercury lamp; most other illuminants do not have sufficient blue and violet rays to give considerable illumination of this color and even if they do, no screen which passes blue and violet is sufficiently opaque to the long waves not to pass enough of them to spoil the effect, if the illuminant is rich in such long waves. The mercury lamp, how- ever, is deficient in these, and thus it is necessary only to blind off the green and yellow rays in order to get the blue and violet light. I show you here a mercury lamp enclosed by a screen consist- ing of a solution of naphtol green (an aniline dye) which transmits only the green light. As you see, in the green the above-described effect does not exist, but the vision is clear, distinct and restful. 27. Beyond the violet the radiation is no longer visible to the eye as light. There is, however, a faint perception of ultra-violet light in the eye, not as distinct light, but rather as an indis- tinct, uncomfortable feeling, some form of dull pain, possibly resulting from fluorescence effects caused by the ultra-violet radiation inside of the eye. With some practice the presence of ultra-violet radiation thus can be noticed by the eye and such light avoided. In the ultra-violet, and possibly to a very slight extent in the violet and even in the blue, a specific harmful effect appears, which probably is of chemical nature, a destruction by chemical dissociation. This effect increases in severity the further we reach into the ultra-violet, and probably becomes a maximum in the range from one to two octaves beyond the violet. These very short ultra-violet rays are extremely destructive to the eye : exposure even to a moderate intensity of them for very few minutes produces a severe and painful inflammation, the after effects of which last for years, and long exposures would probably result in blindness. The chronic effects of this inflam- mation are similar to the effect observed in blue light : inability or difficulty in fixing objects on the sensitive spot F, so that with- out impairment of the vision on the rest of the retina clear dis- PHYSIOLOGICAL EFFECTS OF RADIATION. 53 tinction is impaired and reading becomes difficult or impossible, especially in artificial illumination. It appears as if the sensitive spot F, or the focusing mechanism of the eye, were over-irritated and when used, for instance in reading, becomes very rapidly fatigued and the vision begins to blur. If further irritation by ultra-violet light or by attempting to read, etc., is avoided, gradually the rapidity of fatigue decreases, the vision remains distinct for a longer and longer time before it begins to blur and ultimately becomes normal again. The inflammation of the eye produced by ultra-violet light appears to be different from that caused by exposure to high- power radiation of no specific effect, as the light of a short circuit of a high-power electric system, or an explosion, etc. The main differences are: 1. The effect of high-power radiation (power burn) appears immediately after exposure, while that of ultra-violet radiation (ultra-violet burn) appears from 6 to 18 hours after exposure. 2. The external symptoms of inflammation: redness of the eyes and the face, swelling, copious tears, etc., are pronounced in the power burn, but very moderate or even entirely absent in the ultra-violet burn. 3. Complete recovery from a power burn even in severe cases usually occurs within a few days, leaving no after effects, while recovery from an ultra-violet burn is extremely slow, taking months or years, and some after effects, as abnormal sensitivity to radiation of short wave lengths, may be practically permanent. The general phenomena of a severe power burn are : Temporary blindness immediately after exposure, severe pains in the eyes and the face, redness of eyes and face, swelling, copi- ous tears, etc. These effects increase for a few hours and then decrease, yielding readily to proper treatment: application of ice, cold boric acid solution, etc., and complete recovery occurs within a few days. In chronic cases, as excessive work under artificial illumination, the symptoms appear gradually, but recovery, if no structural changes in the eyes have occurred, is rapid and complete by proper treatment and discontinuance of work under artificial illumination. Most artificial light is given by temperature radiation (incan- descent lamp, gas and kerosene flame), and therefore its radiation 54 RADIATION, LIGHT, AND ILLUMINATION. consists of a very small percentage only of visible light (usually less than 1 per cent), while most of its energy is in the ultra-red and invisible, and for the same amount of visible radiation or light the total radiated power thus is many times greater than with daylight. Regarding chronic " power burn," artificial light, therefore, is much more harmful than daylight, that is, much more energy enters the eye under incandescent illumination than under much more powerful daylight illumination. In a severe ultra-violet burn no immediate symptoms are noticeable, except that the light may appear uncomfortable while looking at it. The onset of the symptoms is from 6 to 18 hours later, that is, usually during the night following the ex- posure, by severe deep-seated pains in the eyes; the external appearance of inflammation is moderate or absent, the vision is not impaired, but distinction made difficult by the inability to focus the eye on any object. The pains in the eyes and head- ache yield very slowly; for weeks and even months any attempt of the patient to use the eyes for reading, or otherwise sharply distinguishing objects, leads to blurring of the vision; the letters of the print seem to run around and the eye cannot hold on to them, and severe headache and deep-seated pains in the eyes follow such attempt. Gradually these effects become less; after some months reading for a moderate length of time during daylight is possible, but when continued too long, or in poor light, as in artificial illumination, leads to blurring of the vision and head or eye ache. Practically complete recovery occurs only after some years, and even then some care is necessary, as any very severe and extended strain on the eyes temporarily brings back the symptoms. Especially is this the case when looking at a light of short wave length, as the mercury arc; that is, there remains an abnormal sensitivity of the eye to light of short wave lengths, even such light which to the normal eye is perfectly harmless, as the mercury lamp. In chronic cases of ultra-violet burn, which may occur when working on unprotected arcs, and especially spark discharges (as in wireless telegraphy), the first symptoms are: occasional headaches, located back of the eyes, that is, pains which may be characterized either as headache or as deep-seated eye ache. These recur with increasing frequency and severity. At the same time the blurring of the vision begins to be noticeable PHYSIOLOGICAL EFFECTS OF RADIATION. 55 and the patient finds it more and more difficult to keep the eye focused for any length of time on objects, as the print when read- ing. These symptoms increase m severity until the patient is obliged to give up the occupation which exposed him to ultra- violet light, and then gradual recovery occurs, as described above, if the damage has not progressed too far. In mild cases recovery from power burns may occur in a few hours and complete recovery from mild ultra-violet burns in a few weeks. Both types of burn may occasionally occur simultaneously and their symptoms then successively. For instance, in a case of an exposure while working for about half an hour with a flame-carbon arc without enclosing glass globe (such an arc contains large amounts of high-power radia- tion, of yellow and orange color, but also a considerable amount of ultra-violet rays), the symptoms of the power burn increased in severity for a few hours, and then rapidly vanished by the application of cold water, and recovery was practically complete six hours after exposure; then some hours later, in the middle of the night, the patient was awakened by severe pains in the eyes, the symptoms of the ultra-violet burn, and had to seek medical attendance. Under proper treatment recovery occurred in a few days, but the blurring of the vision was appreciable for some days longer, and the sensitivity to high-frequency light for some weeks. 28. Arcs produce considerable amount of ultra-violet light,* and in former experiments we have used a high frequency iron arc for producing ultra-violet light and also have seen that even a very thin sheet of glass is opaque for these radiations. For very long ultra-violet rays, that is, the range close to the visible violet, glass is not quite opaque, but becomes perfectly opaque for about one-quarter to one-half octave beyond the violet, and in this first quarter of an octave the harmful effect of the ultra-violet radia- tion is still very small and becomes serious only when approach- ing a distance of about one octave from the visible end of the violet. Clear transparent glass thus offers a practically complete protection against the harmful effects of ultra-violet light, except when the latter is of excessive intensity, and thus arcs enclosed * An arc between silicon terminals emits especially powerful ultra-violet radiation accompanied by little visible light. 56 RADIATION, LIGHT, AND ILLUMINATION. by glass globes are harmless. It is, however, not safe to look into and work in the light of open metal arcs for too long a time. The carbon arc gives the least ultra-violet rays, so little that even without enclosure by glass it is fairly safe; metal arcs give more and the mercury arc gives the greatest amount and reaches to the farthest distance beyond the visible, and these very destruc- tive very short ultra-violet rays have so far only been observed in the radiation of a low temperature mercury arc in a quartz tube : quartz being transparent to these rays while glass is opaque. The high temperature mercury arc in a quartz tube, that is, arc operated near atmospheric pressure as it is used to some extent for illumination, especially abroad, seems to be much less dan- gerous than the low temperature or vacuum arc, but it also requires a protecting glass globe. In general, no metal arc, spark discharge, or glow discharge should ever be used industrially or otherwise without being en- closed by a glass globe, preferably of lead glass, if located so that it may be looked at. Those experimenting with arcs or other electric discharges should always protect their eyes by the inter- position of a glass plate. Thus the sparks of wireless telegraph stations, the discharges of ozonizers, the arcs of nitric acid generators, electric furnaces, etc., may be dangerous without glass enclosure. While artificial illuminants, and especially metal arcs, give an appreciable amount of ultra-violet light, these ultra-violet rays extend only to about one-quarter octave beyond the visible violet and if, as is always the case, the illuminant is enclosed by glass, the harmful effect of these long ultra-violet rays is negli- gible. The radiation of the sun also contains ultra-violet rays, and a larger percentage compared with the total radiation than any glass-enclosed artificial illuminant, and as the light of the sun, that is, daylight, is recognized as perfectly harmless, as far as this specific destructive action is concerned, the same applies to the artificial illuminants, as they contain less ultra-violet rays than the light of the sun. This specific destructive action on the eye of short ultra-violet radiation extends beyond the blank space in the spectrum of radiation (Fig. 14) and still exists, though possibly to a lesser extent, in the X-rays. PHYSIOLOGICAL EFFECTS OF RADIATION. 57 Pathological and Therapeutic Effects of Radiation. 29. Radiation impinging on the tissue of the human body or other living organisms exerts an influence depending on intensity, power and frequency. The effect on the eye has been discussed in the preceding paragraphs. The specific chemical effect in supplying the energy of plant life will be more fully discussed in the following under chemical effects. As is to be expected, the effect of radiation on the living protoplasm of the cells is stimulating if of moderate intensity, destructive if of excessive intensity; that is, by the energy of the radiation the motions of the parts of the protoplasm-molecule are in- creased, and, if the intensity of radiation is too high, the mole- cule thus is torn asunder, that is, destroyed, the living cell killed and inflammation and necrosis (mortification) result. If the intensity is moderate, merely an increase of the rapidity of the chemical changes in the protoplasm, which we call life, results; that is, the radiation exerts a stimulating effect, in- creasing the intensity of life, causing an increased renewal of worn-out tissue and reconstruction, and thus is beneficial or curative, especially where the metabolism is sluggish. Just as in the action on the eye, two different effects probably exist : a general effect due to the energy of the radiation — which with sunlight is a maximum beyond the visible close to the red end of the spectrum, and with most artificial illuminants (those based on incandescence) reaches a maximum still further in the ultra-red — and a specific effect depending on the frequency. The power effect is general and probably fairly uniform throughout the exposed tissue, appears simultaneous with or immediately after the exposure, and thus practically no danger of harmful results from destruction of tissue exists, as excessive intensity makes itself felt immediately, before far-going destruc- tion of tissue can occur, and, therefore, the only possible danger which could exist would be in the indirect effect of stimulation on other organs of the body, as the heart. Thus the use of in- candescent light as stimulant appears fairly harmless. Different is the specific action of high-frequency radiation. This occurs only some time after exposure, from a few hours to several weeks (with X-rays) . As these higher frequencies are not felt by the body as such and exert a powerful action even at such 58 RADIATION, LIGHT, AND ILLUMINATION. low intensities that their energy is not felt as heat, and, further- more, . the susceptibility of different people may be different, there is nothing to guard against excessive and thereby harmful exposure. Furthermore, the damage is far more severe and lasting than with the power effect, and fatal cases have occurred years after exposure. Possibly, as may be expected from selective action, only a few cells in the living tissue are killed by the radiation, and the disintegration products of these dead cells then gradually involve the surrounding living cells, causing their destruction or degeneration, so that the harm is far out of proportion with the immediate destructive effect of the radia- tion proper, especially with penetrating forms of radiation, as X-rays and radium rays, in which the lesions are correspondingly deep-seated. High-frequency radiation (violet, ultra-violet, X-ray) should therefore be used only under the direction of experts fully familiar with their physiological action and danger. The specific action of high-frequency radiation is still absent in the green, begins slightly in the blue and violet, increases into the ultra-violet and persists up to the highest frequencies of the X-rays. It is shared also by the radiation of the radio-active substances, as the alpha and beta rays of radium. While the maximum of this effect probably also lies in the ultra-violet, from one to two octaves beyond the visible spectrum, the effect is profoundly modified by the transparency or opacity of the tissue for different frequencies, and the character of the stimu- lating and pathological effects greatly depends on the depth to which the radiation penetrates the body. The largest part of the organism is water. Water is trans- parent for visible light, becomes more and more opaque in the ultra-red as well as in the ultra-violet, and is again fairly trans- parent for X-rays. Blood is fairly transparent for the long visible rays of red and yellow, but nearly opaque for the shorter violet and ultra-violet rays. Hence next to the X-rays which can pass through the body, the longest visible rays of red and yellow penetrate relatively deepest into the body, though even they are practically absorbed within a short distance from the surface. Thus while the energy maximum of the sunlight is in the ultra-red, the maximum physiological effect probably is that of the red and yellow rays : the same which are the active PHYSIOLOGICAL EFFECTS OF RADIATION. 59 rays in plant life. The violet and ultra-violet rays are absorbed close to the surface of the body by the blood, which is opaque for them. They can thus be made to penetrate deeper — as is done in their therapeutic use — by freeing the tissue of the body from blood by compression or other means. Even then, how- ever, probably only the longest ultra-violet rays penetrate, the very short ones being kept out by the opaque character of the water in the tissue. The penetration of the radiation of the sunlight into the human body is very greatly reduced by acclimatization, which leads to the formation of a protective layer or pigment, more or less opaque to the light. Such acclimatization may be permanent or temporary. Permanent acclimatization has been evolved during ages by those races which developed in tropical regions, as the negroes. They are protected by a black pigment under the skin, and thereby can stand intensities of solar radiation which would be fatal to white men. A temporary acclimatiza- tion results from intermittent exposure to sunlight for gradually increasing periods : tanning, and enables the protected to stand without harmful effects exposure to sunlight which would pro- duce severe sunburn in the unprotected. This acquired protec- tion mostly wears off in a few weeks, but some traces remain even after years. A slight protection by pigmentation also exists in white men, and its differences lead to the observed great differences in sensi- tivity to solar radiation: blondes, who usually have very light pigmentation, are more susceptible to sunburn and sunstroke than the more highly pigmented brunette people. In sunburn we probably have two separate effects super- imposed upon the other: that due to the energy of the solar radiation and the specific effect of the high frequencies, which to a small extent are contained in the sunlight. The two effects are probably somewhat different, and the high-frequency effect tends more to cause inflammation of the tissue, while the energy effect tends towards the production of pigmentation (tanning), and the symptoms of sunburn thus vary with the different pro- portions of energy radiation and of high-frequency radiation as depending on altitude, humidity of the air, the season, etc. 30. The action of radiation on living organisms is stimulating if of moderate intensity, destructive if of high intensity. Thus 60 RADIATION, LIGHT, AND ILLUMINATION. it is analogous with that of any other powerful agent or drug, as alcohol, caffeine, etc. The intensity of light which is destruc- tive to life largely depends on the amount of light to which the organism is accustomed. Those organisms which live in the dark may be killed by an amount of light which is necessary for the life of other organisms. Amongst the saprophytic bacilli, for instance (the germs of putrefaction), many species live in the light, and die, or at least do not multiply, if brought into the dark, while other putrefactive bacilli live in the dark and are killed by light. The latter also is the case with the pathogenic bacilli, that is, the disease germs, as the bacillus of tuberculosis, cholera, etc. As these live in the dark, the interior of the body, they are rapidly killed by light. Light, and radiation in general, therefore is one of the most powerful germicides and disinfect- ants. One of the most effective prophylactic measures, espe- cially against the diseases of civilization, as tuberculosis, etc., thus is to flood our homes with light, especially direct sunlight, while our habit of keeping the light out of our houses by curtains, shades, etc., closing our residences almost light-tight, when leaving them for some time, converts them into breeding places of disease germs, and then we wonder about mortality. Obviously excessive light intensity ultimately becomes harm- ful even to the human organism, and it is therefore advisable to protect ourselves against the light when it becomes annoying by its intensity. It has even been claimed that the impossibility of white men to become permanently acclimatized in the tropics and the change in the temperament of the population of our country within a few generations from their immigration: the increased nervousness, restlessness and "strenuousness," are the result of the greater intensity of the sunlight, especially its high-frequency radiation, compared with the more cloudy climate of our original European home. Whether this is the case remains to be further investigated. It is hard to believe, however, that such a profound effect should result from the exposure of a small part of the body, face and hands, to a more intense light, and the failure of acclimatization in the tropics could well be explained by the higher temperature and its damag- ing effect, while the change from Europe to America is not merely a change from a more cloudy to a more sunny climate, but from a maritime climate, that is, climate having fairly uniform PHYSIOLOGICAL EFFECTS OF RADIATION. 61 and slowly changing temperatures, to a continental climate, with its rapid changes of temperature and enormous temperature extremes, and the difference between continental and maritime climate may be suspected as the cause in the change of the tem- perament of the races. As men have lived for ages in the light, the cells of the human body are far more resisting to the light than the disease germs, which for ages have lived in the dark; and light, and more particularly the high-frequency violet and ultra-violet radiation and the X-rays, thus have found a useful therapeutic application in killing disease germs in the human body. Thus, by expos- ing the diseased tissue to high-frequency radiation, the disease germs are killed, or so far damaged that the body can destroy them, while the cells of the body are still unharmed, but stimu- lated to greater activity in combating the disease germs. As seen, for this purpose the radiation must be of sufficient intensity and duration to kill or damage the bacilli, but not so intense as to harm the cells of the body. Surface infections, as tubercu- losis of the skin (scrofulosis, lupus), thus are effectively and rapidly cured by high-frequency light. More difficult and less certain the effect is if the infection is deeper seated, as then the radiation must penetrate a greater thickness of tissue to reach the bacilli, and is thereby largely absorbed, and the danger thus exists that, before a sufficient intensity of radiation can be brought to the seat of the infection, the intensity at the surface of the tissue may become harmful to the cells of the body. In this case the more penetrating X-rays would be more applicable, as they can penetrate to any depth into the body. They are, however, so far distant in frequency from the light radiation, that the acclimatization of the body to the light radiation probably exists only to a lesser extent against the X-rays; that is, the difference in the destructive effect on the bacilli and on the cells of the body, on which the curative effect is based, prob- ably is less with the X-rays than with the long ultra-violet waves. Since Dr. Finsen introduced phototherapy and radiotherapy, about fifteen years ago, it thus has found a very extended and useful field, within its limitation. This greater destructive action of radiation on micro-organisms than on the cells of the human body, extends not merely to the pathogenic bacilli, but to all organisms living in the dark. 62 RADIATION, LIGHT, AND ILLUMINATION. Thus the spermatozoa — which biologically are independent living organisms — seem to be killed by X-rays before any damage is done to the body, and permanent sterility then results. Amongst the cells of the body differences seem to exist in their resistivity. It is claimed, for instance, that the sensory- nerves are first paralyzed by violet radiation and that intense violet light can thus be used to produce local anaesthesia, suf- ficient for minor operations. Occasionally the effect of light may be harmful in the relation of the human body to invading bacilli. In some eruptive in- fections, as smallpox, ulceration of the skin (leading to mark- ing) seems to be avoided if the patient is kept from the light, and the course of the disease mitigated. As red light, however, seems to have no effect, instead of perfect exclusion of light, which is not very feasible, the use of red light thus seems to offer an essential advantage.