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, i. 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. 



