DEFECTS IN THE OPTICAL APPARATUS 683 



pupil will of course dilate; the pupil of the other eye will also dilate, though 

 unshaded, due to crossed reflex action. 



Defects in the Optical Apparatus. Under this head we may con- 

 sider the defects known as: i, Spherical Aberration; 2, Chromatic Aberra- 

 tion; 3, Astigmatism; 4, Myopia; 5, Hypermetropia. 



The normal or emmetropic eye is so perfect that parallel rays are brought 

 exactly to a focus on the retina without any effort of accommodation, figure 

 466. Hence all objects except near ones (in practice all objects at a distance 

 of twenty feet or more) are seen without any effort of accommodation; in 

 other words, the far-point of the normal eye at rest is at an infinite distance. 

 In viewing near objects we are conscious of the effort (the contraction of 

 the ciliary muscle) by which the anterior surface of the lens is rendered 

 more convex, and rays which would otherwise be focused behind the retina 

 are converged upon the retina. 



Spherical Aberration The rays in a cone of light from a point on an 

 object situated in the field of vision do not all meet in the same point in 

 the retina, owing to the greater refraction of the rays which pass through 

 the margin of a lens than those traversing its central portion. This defect 

 is spherical aberration. In the camera, telescope, microscope, and other 

 optical instruments it is remedied by the interposition of a screen with a 

 circular aperture in the path of the rays of light, cutting off all the marginal 

 rays and allowing the passage only of those near the center. Such cor- 

 rection is effected in the eye by the iris, which forms a diaphragm to cover the 

 circumference of the lens, and prevents the rays from passing through any 

 part of the lens but its center, which corresponds to the pupil. The iris is 

 pigmented to prevent the passage of rays of light through its substance. 

 The image of an object will be most defined and distinct when the pupil is 

 small, if the light is abundant; so that, while a sufficient number of rays are 

 admitted, the narrowness of the pupil may prevent the production of indis- 

 tinctness of the image by spherical aberration. But even the image formed 

 by the rays passing through the circumference of the lens, when the pupil is 

 much dilated, as in the dark, or in a feeble light, may, under certain circum- 

 stances, be well defined. 



Distinctness of vision is further secured by the pigment of the outer sur- 

 face of the retina and of the posterior surface of the iris and the ciliary proc- 

 esses, which absorbs any rays of light that may be reflected within the eye, 

 and prevents their being thrown again upon the retina so as to interfere 

 with the images formed there. The pigment of the retina is especially im- 

 portant in this respect; for with the exception of its outer layer the retina is 

 very transparent; and if the surface behind it were not of a dark color, but 

 capable of reflecting the light, the luminous rays which had already acted 

 on the retina would be reflected again and would fall upon other parts of 

 the same membrane, producing indistinctness of the images. 



