LIGHT OF DIFFERENT WAVE-LENGTHS BY FISH 37 



for the fish, response becomes more accurate as trials with 

 similar brightness are continued. 



4. Intensity Discrimination by Sun?ish as Indicated by Response 



To study the relative sensitivity of sunfish to white 

 light of different intensities, tests were made with some 

 small (4 cm.) untrained sunfish. These fish had shown 

 themselves sensitive to lower intensities of light, than the 

 dace, for they stayed motionless in the farthest corner of 

 the aquarium if brilliantly lighted patches were presented. 

 The stimulus patches were therefore reduced in intensity 

 by the use of slits on both sides. The fish were not fed, 

 but their reactions were observed when the stimulus patches 

 were exposed by lifting the slide door. When one plate 

 was illuminated through a slit of 0.4 mm. and the other 

 through one of 0.6 mm. there were slight differences in 

 behavior toward the two. One record shows a straight 

 return along the midline; another shows that in approach- 

 ing the patches the fish swung back and forth to the mid- 

 line. These turns, doubtless, were responses to a change 

 of stimulus intensity at the mid-line. When one stimulus 

 plate was illuminated through a 0.2 mm. slit and the other 

 plate through slits successively 0.2 mm., 0.25 mm., and 

 0.3 mm., I could determine no difference in the behavior 

 of the fish toward the two plates. The approach to either 

 plate was in a smooth, straight line at these narrow widths 

 of the slit. But when the two slit- widths were 0.2 

 mm. and 0.4 mm., then the approach to the brighter side 

 was no longer straight but slow and wavering. When in 

 this series the slit- width for the brighter plate was increased 

 to 0.6 mm., while the other remained at 0.2 mm., the fish, 

 instead of approaching the plates, lurked in the corner 

 farthest from them. Reference to the graph (fig. 7), which 

 indicates the light intensities reaching the aperture of the 

 lens as varied by slit-widths, shows that with slit-widths 

 0.2 and 0.4 mm. the intensities were about 1 to 2 (as they 

 would be for 2 mm. and 4 mm.). Hence a capacity to 

 respond to intensity differences of white light somewhat 

 greater than a 1 to 2 ratio is present in sunfish. This result 

 was obtained by the method of response. 



