OF SIGHT. 



63 



dioptrical media of the eye are appreciated, but all are not 

 with equal distinctness. Images appear by so much the 

 more indistinct, as they are formed more remotely from the 

 point upon which the optic axis of the eye falls. This point 

 corresponds very accurately to the foramen of Soemmerriug. 

 Whether the peculiar distinctness of vision at this point de- 

 pends on the structure of the retina there, or is to be ascribed 

 to this, that in the usual position of the eyes their axes are so 

 directed towards objects, that the principal rays from these 

 strike through the centres of the lenses, remains doubtful. 

 The latter view is, however, the more probable. For as those 

 rays of a pencil of light that strike through the edges of the 

 lens must be differently refracted from those that pass through 

 its centre ; in consequence of the difference of density between 

 these edges and the centre, &c., they cannot all unite in the 

 same focus ; hence there is unequal dispersion and ill-de- 

 fined images. It is not unimportant to observe, that we 

 do not in fact see more than a single point of an object 

 with perfect distinctness ; if we seem to take in more, 



it is only from the rapidity with which 

 the eyes travel and survey each point 

 in succession one after the other. In 

 surveying a picture closely, we are 

 conscious of this we look at one part 

 after another ; at a distance, indeed, we 

 receive a general impression of the 

 work, but this is only because the rays 

 then come from the object at large hi 

 a pencil so delicate, that it passes en- 

 tirely by the centre of the lens. There 

 is a particular circumscribed spot at the 

 bottom of the eye, corresponding to 

 the place of entrance of the optic nerve, 

 or, at all events, to the centre of this 

 part, which the arteria centralis retinae 

 perforates, where w r e have no sense of 

 visual perception.* 



* Marrotti was the first who described the 

 disappearance of the visual image at the en- 

 trance point of the optic nerve. To make the 

 experiment, let two black objects be taken 

 and placed at a and 6 (fig. 40), upon a white 



Fig. 40. 



