284 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
toads each were tested, making a total of 384 trials in each single pair 
of lights or 2304 trials in all. One of the sets of twelve toads were the 
remainder of the lot used for the first three sets of reactions, when 
both the eyes and skin acted as receptors, though seven individuals 
died during the course of the experiments, and their places had to 
be supplied by seven new toads. The other lot employed was one of _ 
the new lots selected for the last two sets of tests in the same series 
of experiments. 
By referring to Table 8, it will be seen that the results were, in 
general, the same as those obtained when the whole body was exposed 
(Table 7). In any pair of lights there were movements toward both . 
lights, with the larger percentage of responses to the blue, or to the 
light of a given pair which, in the spectrum, is nearer the blue. There 
were only a few more movements to red light, however, in the pairs 
with green and yellow, than there were negative responses to these 
lights, when they were used singly; though, when paired with blue, 
there was 9 % more (Table 2, and Figs. 3 and 4). Red light under 
these circumstances was, therefore, not much more effective than 
darkness in the production of responses. 
When blue and green were paired, the percentage of responses to 
the blue was considerably higher than that to the green, the ratio 
being 62 to 38. But, when blue or green was paired with yellow 
(see also Fig. 6), there was only 4 % more responses to blue than to 
ereen; and when paired with red (see also fig. 4), only 6 % more. 
There was 11 % more responses to blue than to green when they were 
used singly (Table 2). Green also was considerably more effective 
than yellow when these two lights were paired, the percentage of 
responses to them being in the ratio of 66 to 34. But when paired: 
with blue consecutively (see also Fig. 5), the difference in effectiveness 
was only 8 %, and when paired with red (see also Fig. 4), also only 8 %. 
The difference in effectiveness between green and yellow, when used 
singly, was 9 % (Table 2). 
By way of summarizing the results of the experiments with balanced 
pairs of monochromatic lights in which only the eyes served as recep- 
tors, it may be said that they were essentially the same as when both 
the skin and the eyes acted as receptors, and in the main, the same as 
those obtained with single monochromatic lights, when only the eye 
was exposed. Blue, green, and yellow were effective in the production 
of positive responses, but red, in comparison with any other color, 
never showed this. Blue was the most effective stimulus, and red 
the least, and the decrease in effectiveness followed the order of the 
