LAURENS: MONOCHROMATIC LIGHTS. 265 
been made, the light from generator A was screened off, and that from 
generator B was admitted to the dark chamber. The toad was then 
subjected to a second set of four trials, which were carried out in a 
similar way to the first set, except that the toad was rotated counter- 
clockwise. Toad No. 2 was next taken in hand, and the same pro- 
cedure was carried out with it as with toad No. 1, but with this 
difference, that with No. 2 the light for the first four trials came from 
generator B, and for the last four from generator A, instead of the 
reverse. The procedure with toad No. 3 was similar to that with 
toad No. 1, and with toad No. 4 to that with No. 2, and so on 
through the series of twelve toads. In this way the right and left 
sides of the toads were alternately exposed to the lights, and to the 
lights from both generators. It is well known that frogs and toads 
will respond to a sudden turning in one direction by a compensating 
movement of the head in the opposite direction; the method of 
rotation above described offsets any influence of these compensating 
movements upon the reactions. It may be here mentioned that no 
toad was ever under actual experimentation for more than forty 
minutes on any day, and very seldom for more than five minutes 
continuously. 
A period of five minutes was given each toad in which to react. 
If, after this period had elapsed, the toad had shown no response, 
it was rotated through 180 degrees to head in the opposite direction 
for the next trial. The reactions were, however, usually very quick 
and definite. I obtained reactions, occasionally, in less than ten 
seconds after a toad had been placed in position for a trial, though 
the average time of the reactions was a little less than a minute. 
The nature of the reactions was as follows: The toads turned until 
they headed toward or away from the light, and then hopped in the 
given direction. With some toads the whole response of turning and 
hopping was very quickly accomplished, a sudden turn being followed 
by as sudden a hop. In others, there was a sudden turn followed by 
a hop only after an appreciable length of time had elapsed. In still 
others, a considerable period passed before even the turning toward 
or away from the light took place. In a few instances, a slight 
mechanical stimulation, such as a delicate touch from behind, was 
required to elicit the first response, and then the animals eventually 
crawled, rather than hopped. There were a very few cases in which 
toads turned toward or away from the light, but made no further 
movements. All such forms of movement were recorded as either 
positive or negative reactions, but there was still another form of 
