486 GERTRUDE MAREAN WHITE 



Color perception seems to be of some importance in the lives 

 of fishes, since color associations are formed and persist for a 

 considerable time. Such associations may be formed even 

 when the colors are not present simultaneously. The value of 

 such associations to fishes which seek their food in shallow water 

 would be obvious. Food of a particular color once found to be 

 desirable may be singled out and pursued and undesirable sub- 

 stances more easily avoided. 



It is somewhat premature to discuss at length the theory of 

 color vision which the results of these experiments would seem 

 to support. The duplicity theory of von Kries assumes that 

 achromatic scotopic (dark-adapted) human vision is carried out 

 through the mediation of the rods alone, the cones being the 

 organ of photopic (light-adapted) vision. This conclusion is 

 derived from the fact that the fovea of the eye consists entirely 

 of cones, while the extreme periphery contains only rods; the 

 remainder of the retina had both rods and cones. The visual 

 purple is located in the rods. For the light-adapted eye, the 

 fovea is the point of keenest vision, and the brightest part of 

 the spectrum is in the yellow. When the eye is dark-adapted, 

 on the other hand, the fovea is no longer the seat of keenest vis- 

 ion, but instead objects are seen more clearly w^hen focused at 

 points away from the center of the eye. Green appears to the 

 dark-adapted eye as the brightest area of the spectrum and it 

 is in green light that visual purple is most strongly bleached. 

 The theory accounts very well for cases of total color-blindness 

 where the fovea of the eye is affected and the eye is unable to 

 focus strongly on any object. In such instances bright light 

 hinders vision. Night blindness might be explained on the 

 basis of a defect in the rods' interfering with vision in low illu- 

 mination, but leaving vision in light of higher intensity unim- 

 paired. The researches of Hess on the comparative physiology 

 of vision in the lower animals, however, give little support to the 

 duplicity theory, since the eyes of all classes of vertebrates show 

 adaptation to light and darkness, even including tortoises which 

 have neither rods nor visual purple. The results of the experi- 

 ments with the sticklebacks described in this paper seem to have 

 little or no bearing upon this theory. 



