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HANDBOOK OF PHYSIOLOGY. 



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

 A and B are again collected to points a and b; and, if the retina F be 

 situated at a and I, perfect, though reversed, images of the points A and 

 B will be formed upon it: but if the retina be not at a and b, but either 

 before or behind that situation, for instance at H or G, circular lumi- 

 nous 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 denned image will not be formed; or, in other 



FIG. 408. Diagram of the course of a ray of light, to show how a blurred or indistinct image is 

 formed if the object be not exactly f ocussed upon retina. 



words, the rays emitted by a given point of the object will not be col- 

 lected into corresponding point of focus upon the retina. 



DEFECTS IN THE OPTICAL APPARATUS. 



A. Defects in the Refracting Media. Under this head we may 

 consider the defects known as (1) Myopia, (2) Hypermetropia, (3) As- 

 tigmatism, (4) Spherical Aberration, (5) Chromatic Aberration. 



The normal (emmetropic) eye is so adjusted that parallel rays are 

 brought exactly to a focus on the retina without any effort of accommo- 

 dation (i, Fig. 409). Hence all objects except near ones (practically all 

 objects more than twenty feet off) are seen without any effort of accom- 

 modation; in other words, the far-point of the normal eye is at an infinite 

 distance. In viewing near objects we are conscious of an effort (the 

 contraction of the ciliary muscle) by which the anterior surface of the 

 lens is rendered more convex, and the rays which would otherwise be 

 f ocussed behind the retina are converged upon the retina (see dotted lines 

 2, Fig. 408). 



1. Myopia (short-sight) (4, Fig. 409). This defect is due to an ab- 

 normal elongation of the eye-ball. The eye is usually larger than nor- 

 mal and is always longer than normal; the lens is also probably too 

 convex. The retina is too far from the lens and consequently parallel 

 rays are focussed in front of the retina, and, crossing, form little circles 



