648 THE SENSE OF SIGHT. 



and b ; and, if the retina F be situated at a and b, perfect, 

 though reversed, images of the points A and B will be 

 perceived : but if the retina be not at a and b, but either 

 before or behind that situation, for instance, at H or G, 

 circular luminous spots c and /, or e and 0, 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 de- 

 fined 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 

 separately considered under the following heads : i, 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. 



i. Since the retina is concave, and from its centre 

 towards its margins gradually approaches the lens, it 

 follows that the images of objects situated at the sides 

 cannot be so distinct as those of objects nearer to the 

 middle of the field of vision, and of which the images are 

 formed at a distance behind the lens exactly corresponding 

 to the situation of the retina. Moreover, the rays of a 

 cone of light from an object situated at the side of the 

 field of vision do not meet all in the same point, owing to 

 their unequal refraction ; for the refraction of the rays 

 which pass through the circumference of a lens is greater 

 than that of those traversing its central portion. The con- 

 currence of these two circumstances would cause indistinct- 

 ness of vision, unless corrected by some contrivance. Such 



