PHYSIOLOGY OF VISION. 



235 



part of the organ, we shall, in the first instance, suppose a single cone 

 to proceed from a radiant point in the direction of the axis of the eye ; 

 or, in other words, of the antero-posterior diameter of the organ, B b. 



Progress of Luminous Rays through the Eye. 



It is obvious, that the rays which fall upon the transparent cornea 

 can alone be inservient to vision. Those that impinge upon the scle- 

 rotica are reflected ; as well as a part of those that fall upon the cornea, 

 giving occasion, in the latter case, to the image observed in the eye, 

 and to the brilliancy of the organ. Nor does the whole of the cornea 

 admit rays, for it is commonly more or less covered, above and below, 

 by the free edge of the eyelids. Again : the whole of the light, that 

 enters the cornea, does not impinge upon the retina. A portion falls 

 upon the iris, and is reflected back to the eye, in such manner as to 

 give us the notion of the colour of the organ. It is, consequently, 

 the light, which passes through the pupil, that can alone attain the 

 retina. 



Some interesting points of diagnosis are connected with the reflection 

 which takes place from the humours of the eye. If a lighted candle 

 be held before an eye the pupil of which has been dilated by belladonna, 

 and in which there is no obscurity in the humours or their capsules, 

 three distinct images of the flame are perceptible situated one behind 

 the other. Of these images the anterior and the posterior are erect ; 

 the middle inverted. The anterior is the brightest and most distinct ; 

 the posterior the least so. The middle one is the smallest, but it is 

 bright. The anterior erect image is produced by the cornea ; the pos- 

 terior by the anterior surface of the lens ; and the middle or inverted 

 image by the concave surface of the capsule of the crystalline. M. 

 Sanson proposed this catoptric method of examining the eye as a means 

 of diagnosis between cataract and amaurosis, in the latter all the 

 images being seen : and experience has shown it to be a valuable mode 

 of investigating various conditions of the eye, which might not be 

 readily understood without its agency. 1 



1 Gazette Medicate de Paris, 27 Janvier, 1844. See, also, T. Wharton Jones, The Princi- 

 ples and Practice of Ophthalmic Medicine and Surgery, Amer. edit, by Dr. Hays, p. 39, 

 Philad.. 1847. 



