294 Prof. J. Joly on a 



wonderful, however incredible it may seem, that the stimulus 

 arising from one quantum must constitute an appreciable 

 fraction of the threshold stimulus. 



(7) The cones are structurally different from the rods in 

 that they contain no visible quantity of rhodopsin. Further, 

 they differ in that each cone is connected through a separate 

 ganglion cell to the optic nerve. This prevails in the fovea 

 where only cones exist and where colour vision is at its best. 

 Throughout the retina, on the other hand, the rods are 

 grouped ; several individuals contributing their stimuli to 

 the one ganglion cell. This fact is often referred to as 

 accounting for the sensitivity of the foveal area. It is very 

 significant. It reveals an effort of Nature to husband and 

 conserve the cone stimuli and to convey them undiluted to 

 the brain. It suggests that these stimuli are more delicate 

 than those coming from the rods, and are of such a character 

 as to bear no intermingling with other stimuli. 



It is also noteworthy that in the central fovea the cones 

 lose their characteristic conical form. They attain a re- 

 markable length, at the same time diminishing in diameter 

 till the latter sinks as low as one micron. Taking the 

 diameter of the outer segment of the cone at this latter 

 figure, and the length as 45 microns *, we find the surface 

 amounts to 180 times that of the cross section. 



To what is this remarkable effort after surface to be 

 ascribed ? If we assume the light entering the cone at its 

 inner extremity to be uniformly distributed throughout the 

 cone, it must escape laterally with a luminous intensity 

 reduced 180 times. Plainly there is some advantage on the 

 score of sensitivity in this diffusion of the light. In the 

 central fovea we are told that the cones are so closely packed 

 as to take on a prismatic form where the inner segments 

 approximate one to another. The light must, therefore, at 

 least for the greater part, move as I have indicated. 



I assume that the outer segment of the cone is bathed 

 in a photosensitive fluid, probably — almost certainty — 

 rhodopsin. In this the light is absorbed : either directly at 

 the surface of the cone or within the thin layer which separates 

 cone from cone. At the meeting of the cone surface with 

 the sensitiser electrons are emitted. They will also be 

 emitted at such distance from the cone surface as the light 

 can penetrate. Some o£ those freed at the surface enter the 

 nerve with maximum velocity and kinetic energy. Those 

 liberated more deeply enter the nerve with diminished velocity. 



* Greeff's drawing, according- to Schafer, underestimates the length 

 of the cone. On the drawing it is 38 microns. See Quain's 'Anatomy.' 



