IMAGES UPON THE RETINA 



517 



perceiving layer of the retina. Finally, by exact physiological measurements,, 

 H. Miiller has shown that the distance between the vessels and the light- 

 perceiving layer must be from 0.17-0.33 mm., and microscopical measure- 

 ments in turn have shown that the distance (0.2-0.3 mm.) takes us to the 

 layer of rods and cones. Hence it follows with great probability that the latter 

 structures, the rods and cones, are the light-perceiving parts of the retina. 



Why do we not ordinarily see the Purkinje figures? Since the field of vision 

 is always filled by objects which give more or less light to the eye, the pupil may 

 be looked upon as a luminous disk throwing light upon the retina. Now the 

 branches of the central vein of the retina are only about 0.038 mm. in thick- 

 ness; and with a pupillary diameter of 4 mm., the umbra of these branches 

 would be only 0.17 mm. long and so would not quite reach the sensitive layer 

 of the retina. The penumbra which does reach the rods and cones remains 

 always in the same place and we have become so accustomed to its presence that 

 we do not perceive it. In Purkinje's experiment, on the other hand, the shadow 

 falls on an unusual place and the illuminated point has a smaller diameter 

 than the pupil, both of which circumstances tend to favor its perception in con- 

 sciousness. If the source of light be not 

 moved the figure disappears shortly, only to 

 reappear when the source of light is again 

 moved, just as other objects are more read- 

 ily perceived when moving than when at rest. 



Another circumstance which strongly 

 favors the light-perceiving function of 

 the rods and cones is the fact that the 

 other retinal layers gradually thin out 

 toward the yellow spot, so that in the very 

 center of the fovea itself only cone cells 

 are left. These are connected with the 

 other layers of the retina by oblique, 

 lateral branches (cf. Fig. 211). 



Seen from the outside the layer of rods 



and cones forms a mosaiclike surface (Fig. 213), an arrangement well adapted 

 to a light-perceiving function; for every object perceptible to the eye is trans- 

 formed by refraction into a mosaic picture of itself. 



FIG. 213. View of the rods and cones seen 

 from the outer surface of the retina, 

 after Max Schultze. a, arrangement of 

 rods (small circles) and of cones (double 

 circles) in most parts of the retina; 6, 

 arrangement in the region of the mac- 

 ula lutea. 



C. VISUAL ANGLE AND THE LIMITS OF VISION 



When the eye receives light from a luminous point, for whose distance 

 it is not exactly accommodated, the light proceeding from the point is brought 

 to a focus in front of or back of the retina, and an illuminated circular field 

 (dispersion circle) is formed on the retina, the size of which depends upon 

 the location of the fociis. If the focus of the beam is close to the retina, 

 either in front or behind it, the dispersion circle will be small ; if farther 

 from the retina, the circle will be larger. 



All rays which pass through the pupil take a course in the vitreous body 

 as if they proceeded from the picture of the pupil which the lens throws back 



