LAURENS: MONOCHROMATIC LIGHTS. 295 



a greater difference in effectiveness between green and yellow, when 

 received through both the eye and the skin, than when received 

 through only the eye. 



It is clear, therefore, that, while the reactions of the toads to single 

 monochromatic lights, when only the skin was exposed, were, in all 

 essential respects, the same as those when only the eye, or the whole 

 body was exposed, the most effective light — blue — was more effective 

 on the eye than on the skin, as was also the green, but less so than the 

 blue, while the yellow and red showed an almost equal effectiveness 

 through the skin, and through the eye. The difference in the effective- 

 ness of blue and green on the eye and skin was clearly shown when 

 both the eye and the skin acted as receptors. 



When we come to consider the reactions to balanced pairs of mono- 

 chromatic lights, it is found that the relations of the eyes, the skin, 

 and the whole animal, were very much the same as those found in the 

 single lights. The relation of the distribution of effectiveness and the 

 distribution of the several lights in the spectrum was also very similar 

 to that in single lights. The distribution of effectiveness followed 

 closely the distribution of the several lights in the spectrum, neither 

 when one light was paired consecuti\'ely with the others, nor when the 

 pairs of lights were considered according to the distance apart of the 

 lights in each pair in the spectrum. The pairs of lights were arranged 

 in the tables in sequence, according to the distance apart in the 

 spectrum of the lights in each pair, beginning with the two farthest 

 apart, and ending with the two nearest together. But in not one 

 of the three conditions of exposure, did the distribution, according 

 to effectiveness of the more refrangil)le light in each pair, correspond 

 to the order in which the pairs of lights were placed. It came nearest 

 to doing so when only the eyes were exposed. When only the skin 

 was exposed, the lack of stimulating effect of the red light on the skin 

 was, I think, clearly the chief cause of the lack of agreement between 

 the two series of tests. But when both the eyes and the skin were 

 exposed to the lights, though the ineffectiveness of the red light 

 probably in great part explained the lack of agreement between the 

 effectiveness and the distribution of the lights, still, the presence of 

 fewer positive responses to the blue when paired with the yellow, 

 than there were positive responses to the green when paired with the 

 red, was also due, in some part, to the fact that, when used singly, 

 blue light was slightly nearer to green in effect than was yellow to red; 

 and therefore, when blue was paired with yellow, there were fewer 

 responses to blue, than there were to green, when the latter was paired 



