24 CORA D. REEVES 



was a very considerable difference in their intensity. Their 

 similar behavior toward the same spaces now illuminated 

 with red and white might then be regarded as an intensity 

 response. But the occasional peculiar method of ap- 

 proach to the red area was in no way like the behavior 

 toward a dull white plate. It indicated that the fish were 

 reacting to a difference between the quality of the dull 

 white and that of the red. To learn whether the fish were 

 reacting merely to a brightness difference between the 

 white and red patches or, as indicated by their behavior, 

 to a quality difference, a method was sought of equating 

 the brightness of the red and white for the fish. 



A tendency of the fish to rise to the surface of the water 

 in pursuit of floating bubbles and dust had been noted 

 when the red plate was first used in the red-white tests 

 just described. This rising was like that often seen in 

 natural waters on dull days or at sunrise or sunset when 

 the sky is red. It was possible that under the conditions 

 the rising was a response to light of any quality but of a 

 definite brightness for the fish. In that case, if the rising 

 response were shown toward two lights of different quality 

 when presented separately, these might be regarded as 

 of such brightness for the fish as to induce similar reaction. 

 If the fish then showed differential behavior toward the 

 two lights, the response must be based on wave-length, 

 or quality differences. This supposition was tested by 

 using a very dull white plate in place of the red. 



The 5 mm. slit was left above the one plate, which thus 

 remained illuminated with mixed light, while the red filter 

 was removed from above the other plate and a second 

 slit was introduced. This slit was gradually narrowed 

 in successive tests until it was but little more than 0.1 mm. 

 wide, but the rising response was not noted. When the 

 slit reached 0.1 mm., the fish rose to the surface and 

 snapped at small floating objects. Thus while the white 

 light illumination of one of the plates remained constant, 

 i. e., that through a 5 mm. slit, that on the other plate was 

 changed from red without a slit to white with a 0.1 mm. 

 slit. The rising response took place with these two con- 

 ditions of illumination. Apparently the light from the 



