SEC. 1. DIOPTRIC MECHANISMS. 



The Formation of the Image. 



The eye is a camera, consisting of a series of lenses and media 

 arranged in a dark chamber, the iris serving as a diaphragm ; and 

 the object of the apparatus is to form on the retina a distinct 

 image of external objects. That a distinct image is formed on 

 the retina, may be ascertained by removing the sclerotic from the 

 back of an eye, and looking at the hinder surface of the transparent 

 retina while rays of light proceeding from any external object are 

 allowed to fall on the cornea. 



A dioptric apparatus in its simplest form consists of two 

 media separated by a (spherical) surface ; and the optical proper- 

 ties of such an apparatus depend upon (1) the curvature of the 

 surface, (2) the relative refractive power of the media. The eye 

 consists of several media, bounded by surfaces which are approxi- 

 mately spherical but of different curvature. The surfaces are all 

 centred on a line called the optic axis, which meets the retina at a 

 point somewhat above and to the inner (nasal) side of the fovea 

 centralis. In passing from the outer surface of the cornea to the 

 retina the rays of light traverse in succession the cornea, the 

 aqueous humour, the lens and the vitreous humour. Refraction 

 takes place at all the surfaces bounding these several media, but 

 particularly at the anterior surface of the cornea, and at both the 

 anterior and posterior surfaces of the lens. Since the anterior and 

 posterior surfaces of the cornea are parallel, or very nearly so, the 

 rays of light would suffer little or no change of direction in passing 

 through the cornea, if it were bounded on both sides by the same 



