266 HUMAN PHYSIOLOGY 



front of the retina ; hence to unite them on the retina, the refrac- 

 tion of the eye must be decreased by placing before it a diverging 

 [concave] lens. In hypermetropia the retina lies too far forward, 

 and parallel rays meet back of the retina ; in order that in the rest- 

 ing eye they may unite in the retina, the power of refraction must 

 be increased, which is accomplished by the converging [convex] 

 lens. The normal eye is called emmetropic ; in it the second 

 focal point lies on the retina. 



Periscopia is the power of the eye to see clearly points lying far 

 aside from the axis of the eye. The laws thus far enunciated apply 

 only to rays of light which are approximately parallel with the axis and 

 which meet the refracting surfaces nearly at right angles. Rays 

 from luminous points lying far to one side are not accurately united 

 into an image point, but are quite well concentrated at two points 

 on two small lines, of which the posterior one may be regarded as 

 the analogue of an image point. The real image points of lateral 

 and infinitely removed luminous points lie on a curved surface which 

 approximately coincides with the curved surface of the retina. 

 This is true, however, only for the unreduced eye. In the reduced 

 eye the curved surface does not coincide with the retina. 



Imperfection in the dioptric apparatus. 



1. Spherical aberration. If a bundle of light falls obliquely 

 upon a spherical plane separating two media, the peripheral rays 

 are more refracted than the central rays, hence the rays are not 

 accurately united at one point. This is prevented in the eye by 

 the facts that the surface of the cornea is not perfectly spherical, 

 but is more strongly curved in the centre than at the periphery, and 

 that the iris cuts off the peripheral rays. 



2. Chromatic aberration. The various colors of the spectrum 

 are differently refracted, the violet being more strongly refracted 

 than the red ; hence the violet rays are brought to a focus sooner 

 than the red rays. This ordinarily causes no disturbance in vision, 

 but can be observed by covering half of the pupil of one eye with 

 a piece of cardboard and gazing with this eye upon a light object. 

 The borders of the object appear colored because of the chromatic 

 aberration. 



3. Astigmatism. The curvature of the surfaces of separation 

 may be unequally great in various meridians of the surface. Hence 

 the rays of light from a light-bundle falling upon a given meridian 

 will be refracted differently from those falling on another meridian 

 and will unite at another place. Suppose that the meridian having 

 the greatest curvature is at right angles to the meridian having the 

 least^ curvature. It will be found that the cross-section of the 

 bundle of light of parallel rays after refraction forms a straight line 

 at two places, the first of which has the direction of the meridian 



