522 VISION 



anterior focal point of the eye, the rays from it which enter the eye will be 

 parallel after refraction and a shadow of any object (b, in Fig. 217) in the vitre- 

 ous body which is formed on the retina (j3 ) will be of the same size as the 

 object itself. 



B. FORM OF THE REFRACTING SURFACES 



To be able to judge the eye as an optical instrument we must have more 

 detailed information of the actual form of the refracting surfaces. Our knowl- 

 edge along this line is limited for the most part to the cornea, which, however, 



as we have already seen, is the most important 

 of the refracting media (cf. page 513). 



The most exact study of this subject we owe 

 to Gullstrand who used the following method. 

 A disk with concentric circles (Placido's kerato- 

 scope) is so placed as to be reflected by the 

 cornea; the reflected image is photographed by 

 the instantaneous method; and the distances of 

 FIG. 217. After Helmholtz. the circles from one another are measured on 



the photograph. Knowing the corresponding 



differences on the object and the distance of the latter from the cornea, the 

 radius of curvature of the different sectors of the cornea can be calculated. 



We find as a result of this method that the optical zone of the cornea 

 i. e., that part immediately in front of the pupil always approaches the 

 spherical in form,, but that it is often less sharply curved in one meridian 

 than another. Instead of being the segment of a sphere with a circular 

 cross section, it is then a dome with an oval cross section. 



If the surface of the cornea were always perfectly spherical, it would share 

 with all such surfaces the defect of spherical aberration (Fig. 218). It will 

 be evident from the figure that spherical aberration can be corrected by flat- 

 tening the refracting surface at the periphery enough to bring the several 

 foci together. Gullstrand has found from his detailed study of the curvature 

 of the- cornea, that, as a matter of fact, the spherical aberration in the vertical 

 plane is slightly offset by a flattening directly above the line of vision, which 

 is probably due to the pressure of the upper eyelid. Elsewhere the flattening 

 is not sufficient to affect the aberration. Hence we may say that that part 

 of the cornea which is used for direct vision exhibits this defect. 



C. ASTIGMATISM 



When the optical zone of the cornea is not perfectly spherical but is 

 curved more sharply in one meridian than another, the refraction of light 

 will not be equal in the two meridians. If this difference is slight there will 

 be no disturbance to vision; but it not infrequently happens that the asym- 

 metry of structure is great enough to interfere with the ability to focus 

 correctly. 



A beam of light which is not brought to a single point after refraction, 

 but has different focal distances for different meridians, is described as astig- 

 matic. If the two meridians in which the focal distance is greatest and least 



