32 CORA D. REEVES 



c. Tests for the dace H. Blue with red decreasing in 

 intensity. 



The fish H was tested day after day in the same manner 

 as Md (fig. 13, graph H at right), except that the reduc- 

 tions in the width of the sHt, and therefore in the intensity 

 of the red plate, were more rapid. Changes in slit width, 

 cutting off 15 per cent and 50 per cent of the light, did not 

 cause failure to discriminate. When the intensity of the 

 red plate approached the point where it matched the blue 

 in brightness for the human dark-adapted eye (slit width 

 1.5 mm. to 0.9 mm.), the changes in slit were made more 

 slowly. No failure to discriminate resulted, as the plotting 

 of correct choices shows (fig. 13, graph H, red decreasing). 



d. Tests of dace Yl and Y P. Blue with red decreasing 

 in intensity. 



Other dace, Yl and Y P, were introduced into the aquar- 

 ium two months later than H and Md. Their records, also 

 shown in figure 13, are less regular, but they show some 

 points where the discrimination was very accurate at slit 

 widths at which the records for Md and H were not taken 

 for as long a series. 



e. Discussion of blue-red graphs of the dace H, Md, Yl> 

 YP, and conclusions. 



The graphs of these fish, made by plotting the percent- 

 age of correct choices for each successive twenty trials, 

 and distributing these according to the width of slit used 

 with the red variable, are shown in figure 13. The slit 

 widths appear at the top of the graphs, and the corre- 

 sponding relative intensity of illumination of the red filter 

 may be read from the curve, figure 7. 



Each graph falls into two parts, as indicated at the 

 bottom of figure 13. During the first part the red remains 

 at maximum intensity. During the second part the red 

 is slowly decreased in intensity by means of the slit. Each 

 graph (except YP) may also be divided into three parts, 

 determined by the human dark-adapted eye as indicated 

 at the top of the figure. In the first of these the red is 

 brighter than the blue; in the second the region of matched 

 brightness for the human dark-adapted eye is reached; 



