DIOPTRICS OF THE EYE 315 



Optical Defects of the Normal Eye. — The refractive surfaces 

 of the eye exhibit some of the optical defects commonly noticed 

 in lenses, particularly those defects known as chromatic and spherical 

 aberration. White light is composed of ether waves of different 

 lengths and different rapidities of vibration, the shortest waves being 

 those at the violet end of the spectrvim and the longest those at the 

 red end. In passing through a prism or lens these waves are re- 

 fracted unequally and are therefore more or less dispersed accord- 

 ing to the character of the refracting medium. The short, rapid 

 waves at the violet end are refracted the most and are brought to 

 a focus before the longer, red waves, so that the image shows 

 fringes of color instead of being pure white. This phenomenon 

 is known as chromatic aberration. Lenses used for scientific 

 purposes are corrected for this defect or made achromatic by a 

 combination of lenses of crown and flint glass so placed that the 

 dispersive power of one neutrahzes that of the other. The eye 

 exhibits this defect, but not to such an extent as to be no+iceabk 

 in ordinary vision. If, however, an object is in focus when viewed 

 by red light it can be shown that the focus must be changed if the 

 same object is illuminated by violet light. Helmholtz estimates 

 that if the media of the eye possess the same dispersive power as 

 water the rays of violet light must be brought to a focus at about 

 0.434 mm. in front of that of the red rays. 



Spherical aberration depends upon the fact that the rays near the 

 circumference of a lens are refracted more and therefore are brought 

 to a focus sooner than those entering nearest the center. This 

 defect may be noticed in an uncorrected lens by the fact that 

 when the center of the image is in exact focus its margins are 

 slightly out of focus and vice versa. The defect is usually cor- 

 rected, as in photography, by use of a diaphragm to cut off 

 the rays from the periphery of the lens. In the eye both spher- 

 ical and chromatic aberrations are remedied to a large extent by 

 a similar device. The iris constitutes an adjustable diaphragm, 

 which reflex] y narrows as the light increases in intensity and 

 thus cuts off the rays that would go through the periphery of 

 the lens. The interesting physiological control of the movements 

 of the iris is described below. In the eye the defect of spherical 

 aberration is counteracted also by the peculiar structure of the 

 crystalline lens. This organ is composed of concentric layers 

 whose density increases toward the center. The result of this 

 arrangement is that the center of the lens is more refractive than the 

 periphery, and the tendency of the latter portion to refract more 

 strongly is more or less neutralized. 



Abnormalities in the Refraction of the Eye — Ametropia. — 

 The eye that is normal and in which parallel rays focus on the 

 retina when the eye is at rest is designated as emmetropic. Any 



