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 consecutively 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 refrangible 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 
