SENSITIVITY TO WAVE-LENGTH 349 lating value, and that when the blue was decreased greatly in intensity, the habit broke down. The habit likewise dis- integrated when white light was substituted for either of the monochromatic stimuli (see p. 220). Tests upon the rabbit with colored papers show likewise that its responses were probably based upon intensity difference.
Of birds. — When hens are roughly tested by a method similar to the one used on human beings suspected of color defects but with colored grains instead of colored yarns, they pass the test with the ease of the human being normal in color vision. We quote the following test from Hess. It gives the methods and the results.
Upon a dead-black tablecloth 15 by 30 cm. in diameter he lightly fastened yellow-red grains and between these strewed the different grays and greens. On the second day the hen avoided not only the yellow-red grains, but also the red grains and the bluish-red, and pecked only at the grays and greens. This was also the case when he strewed red grains among the greens and the grays (although in this case the red grains were no longer fastened down). With astounding quickness and sureness the animal pecked all the green and gray grains lying between the red grains. If there were only a few of these at hand it hunted all over the entire cloth between the red grains so that after a short interval only the red grains remained untouched. If he brought a hen into the dark room to a row of white rice grains, strewed upon a black background, and colored the grains by means of the spectrum, she here likewise left the yellow-red grains untouched.
He now laid before a so-called red-green blind person (relatively yellow sighted, red-green blind) the differently colored yellow-red and blue-red grains and let him choose out of the pile the grains which for him had the same stimulating value. This subject put yellow-green with yellow-red, pale blue with bluish-red. The dif- ferently colored grains which the hen had differentiated with such sureness were similar or alike to the red-green blind subject. A hen which had been taught to avoid the red grains was fed on a black cloth upon which the green grains were fastened down while the red, orange, etc., were scattered loosely about. After a short time she refused to take the green grains when these were loosely strewn among the others, but did take the red ones. In the spectrum she picked the grains lying in the red end as far as they were visible to the normal human eye. He concludes that the possibility of a red-green blindness in the hens experimented upon is excluded, and that greater strength is given to his contention that their " visual qualities " in this respect are similar to or the same as a normal man's.
K. S. Lashley in an unpublished work finds that when the Indian game bantam is tested with spectral light by the 350 VISION double stimulus method there is good evidence of keen sen- sitivity to wave-length difference. There is a rather wide literature here also on the ability of birds to pick out differently colored papers, glass, foodstuff, etc. While these tests in the case of birds probably do indicate sensi- tivity to wave-length difference, we are never on safe ground in so interpreting them.
Demonstration of the Purkinje effect. — One of the most important facts yet demonstrated concerning the. sensitivity of birds and other animals to wave-length difference is the presence of the Purkinje effect. Several investigators of animal vision agree, some of them without really proving the fact, that the region of greatest stimulating effect shifts with the state of adaptation of the animal and with the de- crease or increase in the intensity of the spectrum. Hess finds that the phenomenon occurs in all of the higher ver- tebrates tested with the exception of the fish. Bauer and Frisch find it even in fish. "While the demonstration of the Purkinje effect cannot be taken as the equivalent of a demonstration of sensitivity to wave-length, it certainly is to be considered as a very positive bit of evidence in that direction, if any parallel can be drawn between animal and human responses. We have in the case of human beings the apparently well supported fact that this effect is present in the case of individuals normal in color vision, but en- tirely absent in those who are defective in color vision.^ On account of the theoretical importance attaching to this phenomenon, K. S. Lashley undertook to test for its pres- ence in some Indian game bantams of pure breed. It will be recalled that on p. 339 the author has shown that the point of highest stimulating value on the threshold curve of the dark-adapted chick lies at 520mM' Lashley, in- stead of trying to obtain a similar curve for the whole spectrum under conditions of light-adaptation, used only ^ It is interesting to note that the presence of the Purkinje effect is apparently not dependent upon the presence of visual purple. The clay birds are almost wholly lacking in rods, and hence in visual purple, and vet it can be shown that the point of highest stimulating effect in the'r spectrum is dependent upon adaptation.
THE PUEKINJE EFFECT 351 the two bands, red and green (and later yellow and blue- green). The chicks were first trained to respond to the more intense of tiuo white lights. In the preliminary training to white light it was shown that when the intensity of the white light contrast is great (ratio of 20 to 1) and the absolute intensity of both is low, the chick is posi- tive to the more intense. "When the intensity of both lights was raised proportionately the positive tendency disappeared. Training was instituted and continued until the animal would respond to the more intense light when the intensities stood in the ratio of about 3 to 1 (absolute intensity was varied from day to day between the limits of 5 to 18 candle meters). Red {650 mm) and green (518/^yM) were then equated in energy and allowed to fall upon the stimulus plates. By a long series of preliminary trials upon thresholds under conditions of light- and dark- adaptation, Lashley finally found a certain intensity rela- tion at which the shift could be obtained. When the green has an intensity of 1/45 the standard (p. 75) and the red 1/6, we obtain the following positive responses: On repetition several days later, but with energy relations slightly different, the following were obtained: Similar tests w^re made v^nth yellow {590 MM) and with blue-green (490yUyw). In a similar wdj it was found that when the yellow was cut to 1/22 of the standard energy and the bluish-green to 1/90, the dark-adapted chick went to the bluish-green 9 times and to the yellow twice; when light-adapted, to the yellow 9 times and to the bluish- green once. This evidence is not wholly conclusive. The number of tests made is small. Nevertheless the results seem to show clearly that the stimulating effect of the blue-green region is heightened by adaptation to darkness.® ® In order that there may be no misunderstanding about the possibility of getting the Purkinje shift with yellow and blue, we 352 VISION Results from a physiological method of testing for color sensitivity. — Rouse made some tests several years ago upon the effect of colors upon the respiration and circula- tion of pigeons. He obtained different types of change in these processes depending upon the color which confronted the animal. His records so far are not to be depended upon, since the light was not in any sense monochromatic nor was its energy at all controlled. If it could be shown that a white light, within a wide range of intensity, pro- duced no marked change in these physiological processes; i.e., if the experimenter could stimulate the animal first with a white light of 1 c.p and then with one of 2 c.p. with- out producing a marked disturbance in such processes (or if it varied in a fixed way with the increase in intensity), while on the other hand if he stimulated them with a red and then with a green light, and such disturbance did not ensue, then we should have to conclude, after proper con- trol, that the wave-length of the light exerts characteristic stimulating effect. The presence of such changes would again be no demonstration of color sensitivity, but such evidence would certainly be supporting.^*^ A similar set of tests should be made upon the pupil, both where the energy carried by the different bands is equal and again where the energy is proportional to its stimulating effect.
In fish. — Investigators do not agree as to the presence of sensitivity to wave-length difference in the fish. Hess give here the complete range of wave-lengths employed in the four stimuli: ^^ Babak has recently made some tests on the sensitivity to colored lights (filters) of frogs from which the forebrain had been removed. The animal to be studied was placed in faint white light for a half hour. At the end of this time the normal breathing rate was de- termined. The frog was then subjected to a given colored light had little stimulating effect, as was also the case with weak mined under each condition. It was found that very intense green light had little stimulating effect as was also the case with weak red light. A very weak violet had, on the contrary, a much greater stimulating effect than the most intense green. A beam of white light as a whole has less effect than the violet constituent of that beam.
SENSITIVITY TO WAVE-LENGTH 353 finds that fish have a spectrum shortene(J at the red end; that they seek always, both in dark- and light-adapted states, the same part of the spectrum, viz., yellow-green to green; i.e., no shift of the point of greatest stimulation is produced by the process of adaptation or by changing the absolute intensity of the spectrum. They behave in this respect as do the totally color-blind human beings who have E to b as the brightest point in the spectrum regardless of its intensity. The view that fish are without sensitivity to wave-length is strengthened by the facts which Hess has brought out by other methods. When offered food on colored or colorless backgrounds of the same white value as the foodstuffs the fish do not respond to it because object and background have the same stimulating values. On the other hand, nearly every other investigator ^^ finds " color " vision in fish. Bauer, e.g., comes to the astonishing con- clusion that fish in the light-adapted state are ' ' terrified ' ' by red (rotscheu) and hence must have color vision when light-adapted. On the other hand, when dark-adapted they behave as though color blind. Hess repeated Bauer's work and was unable to find " red shjmess." Hess had no sooner ended the controversy with Bauer than Prisch's work appeared. The latter investigator has recently stated that fish (Phoxiniis Icevis) are able to '' discriminate " red from all shades of gray and especially from dark gray and black.^- Yellow, green, and blue are likewise *' dis- tinguished " from all shades of gray. Green and blue likewise can be distinguished from one another and from other colors as well. On the other hand, red is confused with yellow. Hess, after the appearance of Frisch's work, retested this species by methods of his own and got only negative results.
Mimicry or adaptation to background no test of color sensitivity. — Within recent years many experiments have been carried out upon mimicry in fish — the tendency for many species of fish to appear like the background upon which they rest. We have described these changes on ^^ Bentley and Washburn, Keighard, Bauer, Frisch, Goldsmith, etc. *^ The honey bee, according to Frisch, is unable to pass this test.
354 VISION p. 124 under the heading of special forms of instinctive response. Although Mast, Frisch, and others maintain that since the eyes are necessary for this reaction, the fish in which such reactions occur must necessarily have color vision, it appears to us that these responses do not neces- sarily have any bearing whatsoever upon the question of color vision as we ordinarily understand that term. Ordi- narily we mean when we say that an animal is sensitive to difference in wave-length that such stimuli play a role in the adjustment of the animal to food, sexual objects, shelter, escape from enemies, etc. I.e., that such stimuli initiate activity in arcs ivJiich end in the striped muscles. It is highly probable that the changes in color involved in mimicry of this kind are controlled entirely by the sym- pathetic system (i.e., through a system of conductors run- ning possibly from some special type of terminal in retina to nucleus of oculomotor and other motor nuclei, thence through white rami to sympathetic ganglia). The recent work of Frisch at least brings out the fact that when the sympathetic nerves are cut the changes in color fail to ap- pear.^^ We may make our point concerning the lack of bear- ing of such changes upon sensitivity to wave-length clearer by saying that we can easily conceive of mimicry of this kind taking place in an animal whose retina does not contain the physico-chemical processes (photo-chemical substances?) necessary to initiate response to differences in wave-length. At any rate the burden of proof falls upon those investi- gators who hold that adaptation to the color of the back- ground necessarily proves that fish are sensitive to wave- length difference.
In reptiles and in amphibia. — No very careful work has been undertaken to test the ability of reptiles to react dif- ferently to visual objects w^hich differ only in wave-length. We have the assertion of Hess that the several species of turtles respond to monochromatic light as does the human ^^ Some experiments made a few years ago upon Anolis, the so- called Florida chameleon, by Charlton, show that the dark-brown state and the pea-green state of that animal are probably induced through the action of the sympathetic system. The receptors involved, however, in this case lie in the skin instead of in the retina.
SENSITIVITY TO WAVE-LENGTH 355 being when the latter is tested with an orange glass in front of his eyes. This assertion is made upon the basis of experi- ments upon the limits of spectral sensitivity and the dis- tribution of stimulating effect in the spectrum rather than upon any clean-cut control work upon the formation of sensory habits. Much the same may be said of the work which has been done upon amphibians. Hess tells us that the Purkinje effect exists in both reptiles and amphibia. But notwithstanding the enormous output from Hess ' laboratory it must be evident even to the casual reader that his results are very superficial in character and leave the problems almost in statu quo. The important results ob- tained.by Gotch upon B. temporaria several years ago have been taken as the equivalent of a demonstration of color vision. Gotch showed, by taking the time-relations of the photo-electric change in the retina, that there are three fundamentally different types of response to colored light: (1) the response to red light is characterized by a long latency of nearly 3/10 second and by its attaining to a considerable maximum, averaging about.0004 volt; (2) to green light by the same short latency as that found in the response to white light, i.e., less than 2/10 second; it is also characterized by its magnitude, the maximum reached averaging over.0005 volt; (3) the response to violet light is characterized by a latency longer than that of the green but shorter than that of the red (25/100 seconds); also by very low intensity, the maximum averaging.00024 volt. We do not see how these results can be interpreted at all until we can carry out the same experiment upon a form in which by other methods color sensitivity has been shown to be lack- ing. It lies well within the bounds of possibility that an ex- cised human eye, from an individual lacking totally in color sensitivity, would yield the same three temporal relations in the photo-electric change and the same differences in poten- tial. If we examine the direct or reflex-like responses of frogs to monochromatic light we find little which bears upon our problem. Such work deals almost altogether with the mechanics of orientation to light. We have learned from it that under certain conditions of experimentation the frog 356 VISION turns away from red light and moves towards blue, and that when red light is admitted at one end of the recep- tacle that contains the animal, and green light at the other, the frog moves from the red end to or towards the green. When red and yellow light are opposed in the same way, movement is from the red to the yelloiv. When red and blue are opposed, movement is immediately towards the blue. In such tests there has been little attempt to control intensity. Usually filters were employed which cer- tainly did not yield monochromatic light. On the whole, it seems safe to say that no single crucial test has yet been made either upon reptiles or upon amphibia which gives positive evidence in support of the view that these forms are sensitive to wave-length difference.
Delicacy of the problems in color sensitivity. — As may be seen from the above survey, we are far from having a satisfactory phylogeny of the color sense. The ideals in color experimentation have not been very high. As one goes through the literature on color sensitivity one is struck by the lack of any critical attitude on the part of investi- gators. Almost any kind of a response to a colored light has been taken as the equivalent of a demonstration of color sensitivity. This lack of a critical spirit may be due in part possibly to the failure to scrutinize carefully the various possibilities of reaction in such stimuli — i.e., the number of factors in them which may afford a basis for re- action. A colored object, paper, light, etc., is a very com- plex stimulus. Whenever a sensory habit is established with respect to such stimuli the animal is certainly being stimulated by one factor or another, but by which one? How can we answer this question until we know a great deal more than we do now about the limits of spectral sensitivity and the relative stimulating value of the differ- ent regions of the spectrum? And by relative stimulating effect here we mean both at low (threshold) intensities and at ordinary intensities. In the case of the chick w^e have the relative stimulating effect at the threshold but w^e do not know it at any other intensity. The relative stimulat- ing effect may be w^holly different at high intensities (re- THE PROBLEM OF COLOR SENSITIVITY 357 gardless of the question whether the animal is normal in its color reactions or abnormal). There is a method of ascer- taining these relations which we pointed out some years ago. It would be possible to start with the threshold intensity of red {660 j^M, e.g.) and then gradually to increase the in- tensity of another red of exactly the same w^ave-length until the animal could just respond positively to the more in- tense, say 70 out of 100 times. We would take this inten- sity (in terms of energy) as a new standard and. deter- mine similarly the next reaction threshold (D.L.). In this way we could lay off ten reaction thresholds at 660 MM- The energy value or its reciprocal of the red at this point would give us our first point on our new curve. In a simi- lar way we would lay off ten such points at each of the other wave-lengths (probably three w^ave-lengths, 660mm, 520 MM) a^d 4:80 MM, would give us a good basis for control work). The curve obtained w^oulcl give us the relative stimulating effect of different regions of a spectrum at inter- mediate intensity. What bearing have such facts upon our color work with animals? Those who have worked at all criti- cally are willing to admit, when red and green have been used, that they have been ignorant in nearly all cases of the fundamental fact as to whether the red was actually stimulating the animal. The habit would arise just as well regardless of whether the (non-sensed) red were the positive color or the negative — i.e., an animal stimulated by green can learn to respond either negatively or positively to it. Similarly when yellow and blue were used as stimuli, the blue may have been outside the range of the animal's spectrum at the violet end. Again, after such habits have been established experimenters have tried to break them down by altering the intensity (also the form, the size, etc.). of one or both stimuli. Now such alterations in intensity cannot be made except in a very unsatisfactory way until we have some clear idea as to the change in the amount of energy which it is necessary to make in order to reverse the stimulating effect (assuming that the animal is color- blind). In order to make our point clearer let us take a supposititious case: Suppose we have made the sensitivity 358 VISION curve (spectrum of medium intensity) suggested above. The energy value at 660/^/^ we will assume to be 400x and at 520/^/^, 20j:. The relative stimulating effect is in the ratio of 20 to 1. Suppose, further, that we have estab- lished such a habit with the green at the energy 20j; and the red at 200x. We now introduce our '' controls," We give the animal a red of greater intensity — a red the energy of which is 300^, or one of less intensity, the energy of which is lOOx. Neither of these would affect the habit. Not until we have run the energy of the red up to 400x will we have offered the (color-blind) animal two stimuli of identical stimulating effect. Nor have we intentionally exaggerated the conditions which we suspect exist. To at- tempt to control the color responses of animals by substitut- ing lighter or darker shades of one of the papers used as a stimulus where the possible range in intensity is so limited is to attempt the impossible. Furthermore, on account of the many possibilities for the entrance of secondary criteria where colored paper are employed, such, e.g., as differences in the texture of the paper and differences in the ironing of the paper, it would seem almost impossible to use such stimuli in the behavior laboratories for anything except preliminary work.
V. Responses to White Light Mammals. — Very few of the mammals have been tested as regards sensitivity to differences in the intensity of white light. The results in our possession come from rough tests made with gray papers and from projected lights (Pawlow's method). Monkeys apparently respond readily to one of two gray cards when the difference in " bright- ness " between them is 9^. They show remarkable readi- ness in picking out foodstuffs on the basis of intensity difference. When tested in the dark room by standard methods the monkey (as is the case with all animals so far tested) shows a surprising lack of readiness to form sensory habits based upon intensity difference. Careful experiments by this method have never been carried very RESPONSE TO WHITE LIGHT 359