602 HANDBOOK OF PHYSIOLOGY. 



in viewing a near object, the accommodation is insufficient to focus 

 them. Thus in well-marked cases distant objects require an effort of 

 accommodation and near ones a very powerful effort. Thus the ciliary 

 muscle is constantly acting. This defect is obviated by the use of convex 

 glasses, which render the pencils of light more convergent. Such glasses 

 are of course especially needed for near objects, as in reading, etc. They 

 rest the eye by relieving the ciliary muscle from excessive work. 



3. Astigmatism. This defect, which was first discovered by Airy, 

 is due to a greater curvature of the eye in one meridian than in others. 

 The eye may be even myopic in one plane and hypermetropic in others. 

 Thus vertical and horizontal lines crossing each other cannot both be 

 focussed at once; one set stand out clearly and the others are blurred and 

 indistinct. This defect, which is present in a slight degree in all eyes, 

 is generally seated in the cornea, but occasionally in the lens as well; it 

 may be corrected by the use of cylindrical glasses (i. e., curved only in 

 one direction). 



4. Spherical Aberration. The rays of a cone of light from an ob- 

 ject 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. This defect is known as spherical 

 aberration, and in the camera, telescope, microscope, and other optical 

 instruments, it is remedied by the interposition of a screen with a circu- 

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

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

 rection is effected in the eye by the iris, which forms an annular dia- 

 phragm to cover the circumference of the lens, and to prevent the rays 

 from passing through any part of the lens but its centre which corre- 

 sponds to the pupil. The posterior surface of the iris is coated with 

 pigment, 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 narrow, the object at the proper distance for vision, and the light 

 abundant; so that, while a sufficient number of rays are admitted, the 

 narrowness of the pupil may prevent the production of indistinctness 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 dark, or in a feeble light, may, under certain cir- 

 cumstances, be well defined. 



Distinctness of vision is further secured by the outer surface of the 

 retina as well as the posterior surface of the iris and the ciliary processes, 

 being coated with black pigment, 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 there formed. The 

 pigment of the retina is especially important in this respect; for with. 



