320 bulletin: museum of comparative zoology. 



boundary of the advancing pigment, a general visual comparison of 

 the slides under the microscope afforded a better basis for judgment 

 than could be obtained by actual measurements. The values given in 

 the columns under BV, G, YG, Y and R (standing for the colors 

 blue violet, green, yellow-green, yellow, and red, respectively) were 

 estimates, on a scale of ten, of the distance the pigment had migrated, 

 regarding the basement-membrane as a starting point and the row of 

 nuclei in the distal ends of the retinular cells as the terminus. Thus, 

 in series 3 the blue-violet occasioned the pigment to migrate the 

 whole distance and was evaluated at 10, while red elicited it for only 

 6 tenths the distance. This series, which grades with respect to 

 influence on migration from blue-violet through green and yellow 

 to red, is photographically reproduced in Plates 2 and 3. In Fig. 3, 

 Plate 2, which exhibits the effect of blue-violet, the pigment is out 

 around the nuclei of the retinal cells; in Fig. 4, Plate 2, (effect of 

 green) it has migrated slightly beyond the boundary (the broken 

 line in left half of photograph) of the tapetal layer, but not so far 

 as the row of nuclei (indicated b^- dotted line); in Fig. 5, Plate 2, 

 (yellow) the pigment and tapetum are coextensive; but in Fig. 6, 

 Plate 3, (red) the pigment, though coextensive with the tapetum at 

 the center of the retina, falls short of the outer boundary of the 

 tapetum on the sides (as the left half of the photograph has been 

 retouched to indicate). All retouching for sake of emphasis, has 

 been restricted to the left half of each photograph. 



The summary of results in the lower portion of Table I shows, 

 reading horizontally, that for BV vs. G, out of 6 trials (a trial being a 

 comparison of two animals in the same series), blue-violet showed the 

 greatest migration 4 times; for BV vs. YG, blue- violet was more 

 effective 3 times, yellow-green once, while twice they tied; for BV 

 vs. Y, blue- violet was more effective 2 out of 3 trials; but for G vs. 

 Y, each evoked more migration that the other 3 times out of 6. In 

 the case of BV vs. R, allowing for the fact that in ocries 7 and 8 certain 

 modifications were made in exposing to red, increasing either the in- 

 tensity or the time of exposure in order to obtain a migration equiva- 

 lent to that with blue-violet, it is maintained that, including these 

 with the other series, the blue-violet outweighed the red in efficiency 

 9 out of 10 times. In addition to this there were six preliminary 

 series in which red failed to evoke as much migration of the pigment 

 as the other colors did. 



The evidence furnished by these observations is conclusive only for 

 the difference in efficiency between the extremes of the spectrum. 



