'84 A NEW OPHTHALMOSCOPE. 



In the camera obscura we have an imitation of the eye, its ground 

 glass screen representing the retina, and its lens — the cornea and lena 

 of the eye. 



If we remove the lens the back of the camera immediately becomes 

 Tisible. 



This phenomenon then can only be explained by the laws of refrac- 

 tion. 



" "When a properly formed eye is exactly accomodated for a lumi- 

 nous object, the diverging rays from this incident upon the eye are 

 refracted by the ocular media in such a manner that they unite at a 

 point in the surface of the retina which is the image of that object. 

 The retina in consequence of its transparency transmits much of this 

 light to the choroid, by which most of it is absorbed ; but many of 

 these rays are reflected in the same direction in which they entered 

 the eye and return to the object whence they started. The object, 

 then, and its image on the retina are reciprocal points (they may be 

 considered conjugate foci) each being in turn object or image."* 

 Thus, let E (fig. I.) represent an eye accommodated for the object O. 

 In this case the diverging rays from O, falling upon the cornea of the 

 eye E, are refracted by the media of the eye and collected at P, a point 

 in the retina of E. This point, P, in E's retina, is the image of the 

 object ; and since the rays, when reflected from the eye, simply 

 retrace their steps, the rays from the retina at P will return only to 

 the object O. These reflected returning rays cannot therefore meet 

 the eye of a person at A, but the pupil of E will appear black. And, 

 if the observer's eye be placed in the line OE the illuminating rays 

 will be intercepted. From this it is apparent that without some 

 special contrivance, one person cannot bring his eye into the direction 

 of the rays returning from the eye under examination, without at the 

 same time intercepting the incident rays. This is effected hy substi- 

 tuting reflected for direct light, the observer placing his eye behind 

 and looking through the mirror into the illuminated eye. This is the 

 principle upon which is constructed the Ophthalmoscope which was 

 invented in 1851 by Helmholtz, a German physiologist, but we are in- 

 debted to Liebreich, also a German, for the convenient little instrument 

 now in general use by Ophthalmoscopists. This Ophthalmoscope, the 

 theory of which is illustrated in fig. II., consists of a metallic mirror 

 1^ inches in diameter and of about 6 inches focal length, pierced by a 



• Htilke, Treatise on the Ophthalmoscope. 



