LAURENS: MONOCHROMATIC LIGHTS. 293 



mitted to the brain, and there call forth different color sensations. 

 Whether these photo-chemical substances exist as well in the skin, 

 it is impossible to say, but, in so far as the reactions of toads are con- 

 cerned, the specific chemical effects of the different wave-lengths upon 

 the nerve terminations in the skin seem to be similar to those upon 

 the nerve terminations in the retina; i. e., the nerve-impulses which are 

 set up by the specific chemical changes due to the effects of the several 

 lights, seem to be similar in the two sets of terminations. Parker 

 (:03, p. 33) expressed the view that "the positive phototropism of 

 eyeless frogs depends upon the capacity of the nervous structures of 

 the frog's skin to be stimulated by light." This capacity for stimula- 

 tion in the nervous terminals of the skin, which seems also to be 

 present in many other amphibians, including the toad (Pearse, :10), 

 must now be extended, as the present work shows, to lights of different 

 wave-lengths. 



The results of the tests of reactions to single monochromatic lights 

 received through the eye, through the skin, and through both organs, 

 have been given together in Table 6. The results in these three cases 

 were substantially the same, with only a few minor points of difference. 

 All the lights produced positive responses in each of the three cases, 

 and the sequence of the lights, as determined by their effectiveness in 

 stimulating the toads, corresponded to their sequence in the spectrum, 

 with blue as the most effective stimulus, and red as the least. Al- 

 though the decrease in the ^ ffecti\'eness of the four lights to call forth 

 positive responses follows the order of the spectrum from blue to red, 

 the distribution of this effectiveness does not correspond very closely 

 to the distribution of the several lights in the spectrum. If each 

 light be designated by the wave-length of its middle band, the position 

 of the four lights in the spectrum would be as follows: blue, 450 /x/x; 

 green, 520 fx/j.; yellow, 595 h/j.; and red, 642.5 fx/j.. The respective 

 distances are therefore: between the blue and green, 70.0 /i/x; between 

 the green and yellow, 75.0 mm; and between the yellow and red, 47.5 MM- 

 It is seen that the yellow and red were the nearest of any pair in the 

 spectrum, but in no one of the conditions of exposure to the light, was 

 this pair of lights the nearest in effect in the production of positive 

 responses. In the reactions in which the eye and skin were both 

 exposed the blue and green were the nearest in effect in the produc- 

 tion of positive responses ; in those in which only the eye was exposed, 

 the green and yellow were the nearest; and in those in which only the 

 skin was exposed, the difference in effectiveness in the production of 

 positive responses was the same between blue and green, and between 

 green and yellow. 



