OPTICAL DEFECTS OF THE EYE 



521 



of the various media all in the same straight line. Strictly speaking, how- 

 ever, this is not the case, for the eye presents a number of optical defects, 

 some of which in the majority of cases are quite negligible, while others 

 occasionally affect its functional power to a very great extent. We shall have 

 space here to discuss only the most important of these defects. 



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A. TRANSPARENCY OF THE MEDIA OF THE EYE 



When we remember how complicated is the structure of the cornea and 

 of the lens, it will not appear strange that these media are found not to be 

 perfectly transparent. If a strong beam of light be thrown into the eye by 

 means of a convex lens, the illuminated part of the cornea and of the lens 

 immediately becomes visible i. e., these structures send out from all points 

 an irregularly diffuse light. This diffuse light 

 likewise passes to the retina, excites it and 

 produces a mist of light within which the 

 images regularly formed on the retina appear 

 enshrouded. With ordinary illumination we 

 do not perceive this mist and it does not inter- 

 fere with vision, but one may be made aware of 

 its existence in the following manner: If in 

 the evening a person direct his gaze away from 

 the artificial light and toward a dark corner, 

 the differences of light and shadow from this 

 quarter are much more readily perceptible than 

 equal differences coming into the eye from the 



direction of the source of light. The reason is that the mist of light thrown 

 into the eye in the latter case by the highly illuminated pupil interferes with 

 the contrast effects necessary to perception of such differences. Accordingly, 

 when one wishes to distinguish slight differences of light and shade, he in- 

 stinctively turns his back to the source of light. 



There also exist in the eye certain flecks which under certain circumstances 

 may interfere considerably with perfect vision, especially if they lie in the 

 posterior part of the vitreous body. The perception of these flecks in the 

 transmitting media is described as " entoptic " phenomena. We have already 

 had an example of such phenomena in Purkinje's figure (page 516). 



Under ordinary circumstances these small dark flecks are not noticed; 

 the reason is that an almost uniform amount of light enters the eye through 

 every portion of the pupil, and thus the entire pupil constitutes the illuminat- 

 ing surface alike for all parts of the posterior portion of the eye. The flecks 

 being smaller than the pupil, the shadows cast by them are naturally very 

 short and do not ordinarily reach the retina. 



FIG. 216. Method of demonstrat- 

 ing entoptic phenomena in one's 

 own eye, after Helmholtz. 



The following method (Helmholtz) may be used for demonstrating these 

 entopic phenomena. A convex lens of large aperture and short focal distance 

 (a, Fig. 216) is placed before the eye; at some distance in front of the lens is 

 placed a candle, b, a small image of which is formed by the lens at its focal 

 point. Then a small screen, c, with a minute opening, is so placed that the 

 reduced image of the flame falls in the opening. If the image lies in the 



