720 HANDBOOK OF PHYSIOLOGY. 



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

 retina; thus the images of distant objects are blurred and indistinct. 

 The eye is, as it were, permanently adjusted for a near-point. Rays 

 from a point near the eye are exactly focussed in the retina. But those 

 which issue from any object beyond a certain distance (far-point} cannot 

 be distinctly focussed. This defect is corrected by concave glasses which 

 cause the rays entering the eye to diverge ; hence they do not come to a 

 focus so soon. Such glasses of course are only needed to give a clear 

 vision of distant objects. For near objects, except in extreme cases, they 

 are not required. 



Hypermetropia (long-sight) (3, fig. 428). This is the reverse defect. 

 The eye is too short and the lens too flat. Parallel rays are focussed 

 behind the retina: an effort of accommodation is required to focus even 

 parallel rays on the retina; and when they are divergent, as in viewing 

 a near object, the accommodation is insufficient to focus them. Thus 

 in well-marked cases distant objects require an elTort of accommodation 

 and near ones a very powerful effort. Thus the ciliary muscle is con- 

 stantly acting. This defect is obviated by the use of convex glasses, 

 which renders 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 stands 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 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. 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 

 diaphragm to cover the circumference of the lens, and to prevent the 

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

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



