CH. LV.] ERRORS OF REFRACTION. 765 



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 Aberi'ation. 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 circular aperture in the 

 path o/ the rays of light, cutting off all the marginal rays and 

 only allowing the passage of those near the centre. Such correc- 

 tion is effected in the eye by the iris, which prevents the rays 

 from passing through any part of the refractive apparatus but its 

 centre. 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 alxrration. 



Distinctness of vision is further secured by the pigment of the 

 outer surface of the retina, the posterior surface of the iris and 

 the ciliary processes, which absorbs the greater part of light 

 that may be reflected within the eye, and prevents its being 

 thrown again upon the retina so as to interfere with the images 

 there formed. 



5. Chromatic Aberration. In the passage of light through an 

 ordinary convex lens, decomposition of each ray into its elementary 

 colours commonly ensues, and a coloured margin appears around 

 the image, owing to the unequal refraction which the elementary 

 colours undergo. In optical instruments this, which is termed 

 chromatic aberration, w corrected by the use of two or more 

 lenses, differing in shape and density, the second of which 

 continues or increases the refraction of the rays produced by 

 the first, but by recombining the individual parts of each ray 



