696 SPECIAL SENSES. 



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 it 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. This correction, however, is not suffi- 

 cient 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 pre- 

 cisely opposite to the aberration of converging lenses. In a compound lens, 

 therefore, it is possible to fulfill the conditions necessary to the convergence 

 of all the incident rays to a focus on a uniform plane, so that the image pro- 

 duced 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 a concave or a plano- 

 concave lens be placed in front of this convex lens, which will diverge the 

 rays more or less, 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, the rays near the 

 periphery, which are most powerfully converged by the convex lens, will be 

 most widely diverged by the peripheral portion of the concave lens ; so that 

 if the opposite curvatures be accurately adjusted, the aberrant rays may be 

 blended. It is evident that if all the rays were equally converged by the con- 

 vex lens and equally diverged by the concave lens, the action of the latter 

 would be simply to elongate the focal distance ; and it is equally evident that 

 if the aberration of the one be exactly opposite to the aberration of the other, 

 there will be perfect correction. Mechanical art has not effected correction 

 of every portion of very powerful convex lenses in this way ; but by a com- 

 bination of lenses and diaphragms together, highly magnified images, nearly 

 perfect, have been produced. Lenses in which spherical aberration has been 

 corrected are called aplanatic. 



It is evident that for distinct vision at different distances, the crystalline 

 lens must be nearly free from spherical aberration. This is not effected by a 

 combination of lenses, as in ordinary optical instruments, but by the curva- 

 tures of the lens itself, and by certain differences in the consistence of differ- 

 ent portions of the lens, which will be fully considered hereafter. 



Chromatic Aberration. A refracting medium does not act equally upon 

 the different colored rays into which pure white light may be decom- 

 posed ; in other words, as the pure ray falling upon the inclined surface of 

 a glass prism is bent, it is decomposed into the colors of the spectrum. As a 

 convex lens is practically composed of an infinite number of prisms, the same 

 effect would be expected. Indeed, a simple convex lens, even if the spherical 

 aberration be corrected, always produces more or less decomposition of light. 



