VI 



DIOPTRIC MECHANISM OF THE EYE 



325 



This optic system, consisting of the refractive media of the 

 observed and observing eye, and a plano-concave lens, may be 

 compared to Galileo's telescope, or to an opera-glass, which consists 

 of an objective formed of a convex lens, and an eye-piece formed 

 of a concave lens. The objective in Helmholtz' ophthalmoscope 

 is represented by the dioptric apparatus of the observed eye, and 

 the eye -piece by the concave lens placed before the eye of the 

 observer. By regulating the distance of the two eyes in ratio with 

 their refractive power, and by bringing the correction lens nearer 

 to or farther from the eye of the observer, it is easy after a few 

 experiments to focus so as to see the details of the retina distinctly. 

 This is facilitated by selecting the concave lenses according to the 



. 149. Diagram of Helmholtz' ophthalmoscope. 



refractive power of the eye under examination stronger (plus) for 

 myopic, weaker (minus) lor hypermetropic eyes. 



Immediately after the invention of Helmholtz' ophthalmoscope, 

 the celebrated oculist Elite described another more practical method, 

 based on the principle of Kepler's telescope, which is a system of 

 convex lenses that form "an inverted, magnified, real image of the 

 object. Elite first employed a concave metal mirror with a central 

 aperture to illuminate the observed eye, instead of Helmholtz' 

 glass plates. Hasner subsequently replaced the metal mirror by 

 one of silvered glass, in which a central aperture alone was left 

 transparent. 



The principle of Elite's ophthalmoscope is shown in Fig. 150. 

 The source of light is placed to one side of the ' observed eye A ; 

 the mirror M, which reflects a large cone of light on to A, is placed 

 somewhat obliquely in front of the eye of the observer B\ a 

 biconvex lens of medium focal length is held close to A, and by 



Y i 



