94 VERTEBRATE PHOTORECEPTORS 



increase in illumination the first cones begin to function, but 

 visual acuity is still mediated by the rods because there are 

 more active rods than cones. Presently, with still further 

 illumination, visual acuity becomes determined entirely by 

 the cones and the gradual augmentation of the number of 

 functional cones will continue until all are active and no 

 further change in visual acuity is possible. The distribution 

 of rod and cone physiological thresholds, assumed by Hecht 

 (1928) to describe quantitatively the data of Koenig, is 

 given in Figure 29. Here it is seen that a few of the cones 

 have thresholds lower than the rods. We see, therefore, that 

 the requirements for the large variations of visual acuity 

 cannot be met entirely by the structural make-up of the 

 retina alone and that they are realized fully by the visual 

 cells possessing different thresholds. Since, however, the 

 resolving power of the retina depends also upon the size 

 and average distance apart of the photosensitive elements, 

 it is apparent that vision must vary greatly in different 

 forms. 



Most teleost fishes have a predominance of rods, which 

 are large, although their lengths vary with illumination. 

 Measurements of the rods of Fundulus and Ameiurus have 

 shown them to be 90-100 ^ long in Ught-adapted eyes and 

 as short as 30-35 m in dark-adapted eyes. In deep-sea fishes 

 where one would expect rods to prevail, they apparently 

 are fewer (Karsten, 1923) or are lacking altogether (Brauer, 

 1908). Many teleost cones are also large, but there are 

 exceptions as there are with the rods. In order to compare 

 the relative sizes of the rods and cones in the various verte- 

 brate groups the reader is referred to plates I and II from 

 Arey (1928) where the visual cells pictured have been drawn 

 to a uniform magnification. Whereas, in general, the visual 

 cells in fishes are large, those of the dim-light fishes are very 

 small. One can say, however, that fishes in general possess 

 visual cells, particularly the rods, which are distinctly larger 

 and farther apart than in mammals. The same can be said 

 of amphibians as far as rods are concerned. The frog has 



