26 OLD FLIES IN NEW DRESSES 



plays an important part in the visual power 

 of the trout, particularly, I should think, in 

 a dim light. 



The fact that the rods are absent from 

 the trout's retina does not bear the im- 

 portant significance that one would 

 imagine on first realising it. The fovea 

 centralis of the human retina is the seat 

 of most acute vision, and in the fovea 

 centralis there are no rods. The cones in 

 the retina of the trout are very closely 

 arranged, so that they are practically in 

 contact with each other, and their outer 

 limbs are rather longer and finer than in 

 the case of man. This layer of cones 

 extends to the periphery of the retina, and 

 the cones are just as closely arranged as 

 far as they extend. These facts should 

 lead us to believe that the vision of the 

 trout is probably extremely acute, in fact, 

 as we find in the retina of the trout, no 

 material difference from the fovea centralis 

 of the human retina, we have no reason 

 to suppose that the visual powers of the 

 whole of the retina of the trout, should 

 differ in any way from the visual powers 

 possessed by the fovea centralis, the seat 



