340 PROCEEDINGS OF THE AMERICAN ACADEMY. 



animals which normally react positively to light, and also those which 

 are negative, were used in order to determine whether, as would be 

 expected a priori, the responses (evidences of discrimination between 

 the two lights) were reciprocal in the two cases. Forms which would 

 require to be experimented with under water were avoided on account 

 of the practical difficulties involved, — not only optical complications 

 due to the reflection and refraction of light by the water and the 

 containing vessel, but also on account of the difficulty of directly test- 

 ing the lights at the middle point in order to determine whether that 

 received from one source was exactly equal to that received from the 

 other. 



Before passing on to a description of the apparatus most used, I wish 

 to express my indebtedness to Professor G. H. Parker, to whom, as has 

 already been mentioned, the study owed its inception, and under whose 

 direction, in connection with one of his courses, it was carried on for a 

 year. The later prosecution of the work was under the direction of 

 Professor E. L. Mark ; to him I owe my gratitude for much valuable 

 criticism and advice, since to his attention to accuracy of method both 

 in experimentation and in the deduction of conclusions therefrom must 

 depend in large part any merit which the present contribution may 

 possess. 



II. Description of Apparatus. 



In all the experiments on the reactions of animals to two lights of 

 different areas, as well as in certain of the other lines of investigation^ 

 the same general apparatus was used. Special devices were found 

 necessary in working with each species of animal ; these can best be 

 explained in connection with the accounts of experiments with the 

 respective species. Although the apparatus was changed in some of 

 its details and added to from time to time, the changes and additions 

 were small and comparatively unimportant, the general plan and ar- 

 rangement of the apparatus remaining essentially the same throughout. 

 It will be described in its final form, reference being made to such addi- 

 tions or alterations as seem to be worthy of mention. 



The apparatus was installed in a long, narrow room (7 meters by 

 about 2 meters) commonly used for photographic purposes. The single 

 window of this room could conveniently be made light tight, while an 

 antechamber at the entrance provided against the admission of stray 

 light at that end of the room. The side walls were of brick, the rough- 

 ness of which tended to give less definite reflections than smoother 

 surfaces would have done, and the whole interior of the room was 

 painted dead black. Furthermore, practically everything in the room. 



