508 



THE SENSE OF SIGHT. 



before the eye are first refracted, that is, are bent towards the 

 axis of the cone, by the cornea c c, and the aqueous humor 

 contained between it and the lens. The rays of each cone are 



FIG. 184. 



again refracted and bent still more towards its central ray or 

 axis by the anterior surface of the lens E E ; and again, as 

 they pass out through its posterior surface into the less dense 

 medium of the vitreous humor. For a lens has the power of 

 refracting and causing the convergence of the rays of a cone 

 of light, not only on their entrance from a rarer medium into 

 its anterior convex surface, but also at their exit from its pos- 

 terior convex surface into the rarer medium. 



In this manner the rays of the cones of light issuing from 

 the points A and B are again collected to points at a and b; and 

 if the retina F be situated at a and 6, perfect, though reversed, 

 images of the points A and B will be perceived ; but if the ret- 

 ina be not at a and b, but either before or behind that situa- 

 tion for instance, at H or o circular luminous spots c, and /, 

 or e and o, instead of points, will be seen; for at H the rays 

 have not yet met, and at G they have already intersected each 

 other, and are again diverging. The retina must therefore be 

 situated at the proper focal distance from the lens, otherwise a 

 defined image will not be formed ; or, in other words, the rays 

 emitted by a given point of the object will not be collected 

 into a corresponding point of focus upon the retina. 



The means by which distinct and correct images of objects 

 are formed in the retina, in the various conditions in which 

 the eye is placed in relation to external objects, may be sepa- 

 rately considered under the following heads: 1, the means 

 for preventing indistinctness from aberration ; 2, the means 

 for preventing it when objects are viewed at different distances; 

 3, the means by which the reversed image of an object on the 

 retina is perceived as in its right position by the mind. 



