LAURENS: MONOCHROMATIC LIGHTS. 299 
alone, for all the lights used in these experiments were very closely 
equal in the energy they contained. The different effectiveness of 
the lights must, therefore, depend primarily upon the wave-lengths, 
and upon the chemical substances on which the lights acted. These 
substances are generally assumed to be of several kinds, and conse- 
quently it would be natural to expect that light should affect them 
differently. The degrees of effectiveness of the several lights for the 
eye and the skin were, however, remarkably uniform. 
Sensitiveness to differences in wave-lengths, 1. e. to color, was there- 
fore present in the skin, as well as in the eyes of toads, though 
somewhat reduced in the former. Blue light was the maximum stimu- 
lus in the production of responses, the other lights forming a decreasing 
series, until in the red the effect was hardly more than that of dark- 
ness. The effect of each light was specific, and due, probably, to 
specific chemical changes caused by each set of wave-lengths. These 
specific chemical changes depended primarily upon the wave-lengths, 
and secondarily upon the absorption of light, the energy content of 
the several lights playing no part. 
V. Summary. 
1. Sensitiveness to differences in wave-lengths is present in the 
skin of toads, as well as in their eyes. 
2. Blue light is the most effective stimulus in the production of 
responses, while green, yellow, and red form a decreasing series, 
corresponding only roughly to their relative positions in the spectrum. 
3. Red light, when used singly, is not much more effective than 
darkness in the production of responses, and when paired with other 
lights, this slight effectiveness is even more decreased. 
4. The sensitiveness of the skin to differences in wave-lengths is 
less than that of the eyes. When single lights were used, there were 
more negative responses when the skin only was exposed, than when 
the eyes, or the eyes and the skin together, were exposed. In balanced 
lights there were more movements toward the less refrangible light 
of a given pair when the skin only was exposed, than in the other two 
conditions of exposure, except in the pairs of red with green and with 
yellow. 
5. The reactions when the whole animal was exposed to the light 
showed the influences of the slight differences in sensitiveness of the 
eyes and of the skin. 
