STUDIES IN THE RETINA. 25 



are two different kinds of simple cones.^ These as usually 

 seen are shown in fig. 1. The one is short without a basal 

 vacuole, and the other about one third longer and with a basal 

 vacuole, which is always large, and may at times be enormously 

 distended. It is somewhat remarkable that I have not yet 

 seen any notice of these differences in previous works ; but the 

 literature on the subject is so vast that I make no pretence of 

 having waded through more than a very small portion of it. 



The real significance of these differences is, however, better 

 seen when the distal portions, now described and figured for 

 the first time, are added to them. 



The two types of cone thus revealed are shown in figs. 

 2 (cf. 6, d, f, g, with h, i) and 3 (cf. a, h, c, with d, e, h, i), 

 and diagrammatised in fig. 4, C2 (cf. with C3, cj. Their differ- 

 ences can best be seen if they are drawn upon a scale. Careful 

 measurements show that each cone may be divided into nearly 

 equal sections — at least in their proximal halves — as indicated 

 on the scales in figs. 2 and 4. In type C2, fig. 4, the first section 

 extends from the memb. limitans ext. to the spherical refrac- 

 tive globule which is characteristic of the cones of the frog. 

 The second section extends from this globule to a point 

 where the structure usually breaks in ordinary preparations. 

 There is generally here a minute vacuole or a sudden thinning 

 or change in the intensity of the staining. Beyond this it is 

 doubtful whether any divisions really exist ; the fineness of 

 the thread, and its great delicacy and liability to be violently 

 affected by reagents, render it impossible to say for certain 

 whether the appearances can always be relied on. I have 

 certainly at times seen what appeai"ed to me to be other 

 divisions (see figs. 2, / and g, and 3, a and c) . At the very 

 tip of the cone there appears to be a terminal vesicle, to 

 which our attention will presently have to be directed. 



In the cones of the second type (fig. 4, Cg and c^) the first 

 section can be easily distinguished from the second. It 



' What are known as double cones will be mentioned later, and are not 

 referred to here. 



