LAURENS: MONOCHROMATIC LIGHTS. ava 
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 by 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 very 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 
