LAURENS: MONOCHROMATIC LIGHTS. 265 



been made, the light from generator A was screened oflF, 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 



