LAURENS: MONOCHROMATIC LIGHTS. 271 



toads each were selected. These tests were carried out, therefore, 

 on thirty-six separate toads. It should be mentioned that the first 

 series of reactions of hooded toads which were recorded were so irregu- 

 lar that I was led to believe that the skin of the toad was not sensitive 

 to differences in wave-lengths. This was absolutely disproven by the 

 later series, as will be seen by referring to Table 3. The irregularity 

 in the reactions of the first set were probably due to the irritation 

 caused hy the hoods. This first series of trials, which was carried 

 out relatively soon after the hoods had been placed on the toads, but 

 after they had apparently become used to them, were therefore thrown 

 out, and in all the later experiments a toad was not tested until it 

 had undergone the experience of wearing a hood for a few hours every 

 day for a week. Moreover, in the immediate preparation for the 

 experiments, the twelve toads were hooded the afternoon before the 

 tests, and were left so in the experimental dark-room over night. On 

 two or three occasions, a toad was found in the morning with the 

 hood off. In such cases the hood was replaced, exposing the toad as 

 little as possible to light, and this animal was not tested until it had 

 been for at least 45 minutes in the dark. 



It will be seen, by referring to Table 3, that the reactions of toads, 

 when only the skin was exposed, were in general the same as when 

 the whole body, or only the eye, was exposed. All the lights produced 

 positive responses, but the effectiveness of the blue was the greatest, 

 and that of the other lights decreased in the order of the spectrum. 

 The green light, however, was not vevy much less effective than the 

 blue, there being a difference in the number of reactions of only 6 % 

 between them ; nor was the yellow much less effective than the green, 

 the difference being again only 6 %. The red was clearly less effective 

 than the yellow, there being only 57 % of positive responses in this 

 light, a decrease of 8 % from that in yellow. Red light on the skin, 

 therefore, is more effective than darkness in the production of photo- 

 tropic responses, though not much more so. The nature of the 

 reaction was the same as when the light was received through both 

 the eye and the skin, except that the turning of the toad was followed 

 in most cases by crawling, rather than by hopping, though the latter 

 method of locomotion was by no means unusual. 



By way of summarizing the results of the experiments with only 

 the skin as the receptor, it may be stated that they were in all essential 

 respects the same as when both the eye and the skin acted as receptors, 

 and as when the eye only acted as the receptor. Blue light was the 

 most effective stimulus, but green was not much less effective than 



