46 CORA D. REEVES 



b. Lenses. — At the close of the red-blue series I slipped 

 •out the lenses and tested the fish with only the red and 

 blue filters in place. This reduced the intensity of the red 

 and blue plates and changed the distribution of the light. 

 Although the fish acted uneasy, they continued to dis- 

 criminate. 



c. Filters. — Because the blue filter showed some irregu- 

 larities in transmission, which might be the sign by which 

 the fish knew where to come for food, a new filter was 

 obtained. The red filter was changed several times, but 

 these changes did not affect behavior. 



d. Slits. — In the experiments described a slit was used 

 to regulate the light on the red side only. A mirror was 

 used to determine the distance at which the image of the 

 slit on the cylindrical lens was visible. The curtain already 

 mentioned was used to hide the lenses from the fish (p. 14.) 



After the experiments had been completed a further 

 test was made on untrained fish to learn whether by any 

 possibility the difference in the appearance of the lenses 

 could have influenced the behavior of the fish. This con- 

 sisted in extending the vertical partition C, fig. 1, to the 

 doorway D. The fish thus chose while still within the 

 discrimination compartment at a point from which there 

 was no possibility of their seeing the lenses. It was found 

 that the fish could be trained to go to the blue plate after 

 the partition was introduced. The fish had not been 

 guided by the reflection of the slit on the lens above the 

 red stimulus patch. 



As a further check a suspension of a very fine bone-black 

 in a glycerine-water mixture was substituted for the slit. 

 This suspension in different concentrations was placed in 

 a glass-bottomed container above the red plate. By this 

 arrangement there was no slit on either side. The blue 

 patch remained rectangular and sharp bordered, but, 

 owing to reflections from the carbon particles, the borders 

 of the red patch were now indefinite. The small sunfish 

 swam up to the blue at all intensities of red, including 

 those which matched the blue for the human dark-adapted 

 eye, as did also the dace H . The dace 17 circled about and 

 gave indefinite responses, while YP went to the red plate. 



