LAURENS: MONOCHROMATIC LIGHTS. 279 
of the middle bands among these being 75.0 uy, 70.0 wu, and 47.5 up, 
respectively. 
Five sets of twelve toads each were tested in each of the six double 
pairs of lights, and since each toad was given eight trials in each com- 
bination of lights, the total number of trials for the five sets was 
960 in each single pair of lights or 5760 trials in all. Three of the five 
sets of twelve toads were the same as were used in testing the reactions 
to single monochromatic lights, when both the eye and the skin acted 
as receptors. Two new lots of twelve toads each were, however 
Fic. 3. Curves representing the percentages of negative responses to the four sets of 
wave-lengths, when received through both the eye and the skin ( Nis 
through the eye only (----); and through the skin only (......). Wave- 
lengths as abscissae and precentages of negative responses as ordinates. Points 
marked on axis of abscissae are the positions of the wave-lengths of the middle 
band of each light. B = Blue; = Green: f= Red) Y = Yellow? 
selected for the other two sets. These tests were, therefore, carried 
out on 36 separate toads. 
It will be seen, by referring to Table 7, that the results of the tests 
of the reactions to balanced pairs of monochromatic lights were, in 
the main, very similar to those obtained in response to single mono- 
chromatic lights under the same conditions. The numbers under 
each pair of lights represent the combined results of two sets of trials, 
