836 THE EYE AND VISION JCH. LVIII. 



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 circular 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 correction 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 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 narrow- 

 ness of the pupil may prevent the production of indistinctness of 

 the image by spherical aberration. 



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 most of the light which is 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, is 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 into its original white 

 light, corrects any chromatic aberration which may have resulted 

 from the first. It is probable that the unequal ^refractive power of 

 the transparent media in front of the retina may be the means by 

 which the eye is enabled to guard against the effect of chromatic 

 aberration. The human eye is achromatic, however, only so long as 

 the image is received at its focal distance upon the retina, or so 

 long as the eye is properly accommodated. If these conditions 

 are interfered with, a more or less distinct appearance of colours is 

 produced. 



From the insufficient adjustment of the image of a small white 

 object, it appears surrounded by a sort of halo or fringe. This 

 phenomenon is termed Irradiation. It is partly for this reason that 



