SPHERICAL ABERRATION. 81 



the retina ; a few, only, of the rajs 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. For example, in 

 examining an object with an imperfectly-corrected objective 

 under the microscope, 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 difficulty may be 

 in part corrected if the rays of light be 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 corrects the spherical aberration of the crystalline in 

 part, by eliminating a portion of the rays that would other- 

 wise fall upon its peripheral portion. But this correction 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 which diverge the rays 

 of light is precisely opposite to the aberration of converging 

 lenses. If, therefore, we construct a compound lens, 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 innumerable focal points situated in different 

 planes. The fact that but a few of these focal points are in 

 the plane of the retina renders the image indistinct. If we 

 place behind this convex lens a concave lens, by the action 

 of which the rays are more or less diverged, the inequality 

 of the divergence by different portions of the second lens 

 will have -the following effect : As the angle of divergence 

 gradually increases from the centre toward the periphery^ 



