FORMATION OF IMAGES IN THE EYE. 85 



first made by Purkinje : * We concentrate upon the sclerotic, 

 with a convex lens of short focus, an intense light, at a point 

 as far as possible removed from the cornea. This passes 

 through the translucent coverings of the eye at this point, 

 and the image of the light reaches the retina. If we then 

 look at a dark surface, we have the field of vision presenting 

 a reddish-yellow illumination, with a dark, arborescent ap- 

 pearance produced by the shadow of the large retinal vessels ; 

 and, as we move the lens slightly, the shadow of the vessels 

 moves with it. Without going elaborately into the mechan- 

 ism of this remarkable phenomenon, it is sufficient to state 

 that Heinrich Miiller a has arrived at an absolute mathemati- 

 cal demonstration that the shadows of the vessels are formed 

 upon the layer of rods and cones, and that this layer alone 

 is capable of receiving impressions of light. His explanation 

 is accepted by all writers at the present day, and is regarded 

 as positive proof of the peculiar sensibility of this portion of 

 the retina. In carefully-conducted observations of this kind, 

 a spot is seen in which no vessels appear, which corresponds 

 to the fovea centralis. When the experiment is prolonged, 

 the vessels disappear, as the sensibility of the retina becomes 

 diminished by fatigue. 8 



Theoretically, an illuminated object placed in the angle 

 of vision would form upon the retina an image, diminished in 

 size and inverted. This fact is capable of actual demonstra- 

 tion. In white rabbits and other albinos, Magendie has been 

 able, by removing the fat and muscles covering the posterior 

 portion of the eye, to see the image of a flame upon the reti- 

 na, inverted and diminished. 4 By an ingeniously-arranged 



1 PURKINJE, Beitrage zur Kenntniss des Sehens in subjectiver Hinsicht, Prag, 

 1819, S. 89, et seq. 



2 HEINRICH MULLER, Anatomisch-physiologische Untersuchungen uber die Reti- 

 na, Leipzig, 1856, S. 10*7, et seq. 



3 HELMHOLTZ, Optique physiologique, Paris, 1867, p. 214, et seq. 

 Helmholtz gives a very full explanation of these phenomena and of their 



physiological importance. 



4 MAGENDIE, Precis elementaire de physiologic, Paris, 1836, tome i., p. 79. 



