6/2 SPECIAL SENSES 



lenses before the eyes. In old age the crystalline lens becomes flat- 

 tened, its elasticity is diminished and the power of accommodation is les- 

 sened ; conditions which also tend to bring the rays to a focus behind 

 the retina. This condition is called presbyopia. To render near vision, 

 as in reading, distinct, objects are placed farther from the eye than 

 under normal conditions. The defect may be remedied, as in hyperme- 

 tropia, by placing convex lenses before the eyes, by which the rays are 

 converged before they fall on the lens. 



Spherical Monochromatic Aberration. In a convex lens in which 

 the surfaces are segments of a sphere, the rays of light from an object 

 are not converged to a uniform focus, and the production of a distinct 

 image is impossible. For example, if the lens had regular curvatures, 

 the rays refracted by its peripheral portion would be brought to a focus 

 in front of the retina ; the focus of the rays converged by the lens near 

 its centre would be behind the retina ; a few, only, of the rays would 

 have their focus at the retina itself ; and as a consequence, the image 

 would appear confused. This is illustrated in imperfectly-corrected 

 lenses, and is called spherical aberration. It is also called monochro- 

 matic aberration, because it is to be distinguished from an aberration 

 that involves decomposition of light into the colors of the spectrum. If 

 an object is examined under the microscope with an imperfectly-corrected 

 objective, it is evident that the field of view is not uniform and that 

 there is a different focal adjustment for the central and the peripheral 

 portions of the lens. In the construction of optical instruments, this 

 error may be in part corrected if the rays of light are cut off from the 

 periphery of the lens by a diaphragm, which is an opaque screen with 

 a circular perforation allowing the rays to pass to a restricted portion 

 of the lens, near its centre. The iris corresponds to the diaphragm of 

 optical instruments, and it corrects the spherical aberration of the lens 

 in part, by eliminating a portion of the rays that would otherwise fall 

 upon its peripheral portion. This correction, however, is not sufficient 

 for high magnifying powers ; and it is only by the more or less perfect 

 correction of this kind of aberration by other means, that powerful lenses 

 have been rendered available in optics. 



The spherical aberration of lenses that diverge the rays of light is 

 precisely opposite to the aberration of converging lenses. In a com- 

 pound lens, therefore, it is possible to fulfil the conditions necessary to 

 the convergence of all the incident rays to a focus on a uniform plane, 

 so that the image produced behind the lens is not distorted. Given, for 

 example, a double-convex lens, by which the rays are brought to innu- 

 merable focal points situated in different planes. The fact that but few 

 of these focal points are in the plane of the retina renders the image 



