1148 



FORMATION OF RETINAL IMAGES. [BOOK m. 



for practical purposes, no serious error is introduced, if instead of 

 two such surfaces we assume the existence of one surface lying 

 midway between the two. In this way we arrive at the 'reduced 

 diagrammatic eye/ or 'the reduced eye' as it is called, in which 

 the several surfaces and media of the actual eye are replaced by 



FIG. 136. DIAGRAM OF THE SCHEMATIC OB DIAGRAMMATIC EYE. 



one (ideal) spherical surface (Fig. 136, P), having one nodal 

 point, N ; the two media which the surface separates are supposed 

 to be air on the one side and water on the other. 



The several positions of the cardinal points of this 'reduced 

 eye ' are as follows : 



The principal point, where the one surface of the system cuts 

 the optic axis, lies in the aqueous humour, 2 '3448 mm. behind the 

 anterior surface of the cornea. 



The nodal point lies in the back part of the lens, *4764 mm. 

 in front of the posterior surface of the lens. 



The posterior principal focus lies 22*647 (22*819) mm. behind 

 the anterior surface of the cornea, that is to say, practically lies on 

 the retina. 



The anterior principal focus lies 12*8326 mm. in front of the 

 anterior surface of the cornea. 



The radius of curvature of the (ideal) surface is 5*1248 mm. ; 

 (that of the cornea is 8 mm. and of the anterior surface of the 

 lens 10 mm.). 



706. By help of this ' reduced eye ' we are enabled to trace 

 out the paths of rays of light through the actual eye, and to study 

 the formation of images on the retina. When an image of an 

 external object, such as an arrow (Fig. 137), is formed in such an 

 eye, each point of the object is considered as sending out a pencil 



