CH. LVI.] REFRACTION OF LIGHT 779 



suggested by Listing as the reduced eye, has the following dimen- 

 sions : 



From anterior surface of cornea to the principal point = 2 '344 3 mm. 



From the nodal point to the posterior surface of lens = '4764 



Posterior chief focus lies behind cornea . . . = 22-8237 



Anterior chief focus in front o? cornea . . . = 12-8326 



Radius of curvature of ideal surface . . . = 5-1248 



The term index of refraction means the ratio of the sine of the 

 angle of incidence to that of the angle of refraction ; this is explained 

 in the small text beneath fig. 579. 



In this reduced or simplified eye, the principal posterior focus, 

 about 23 mm. behind the spherical surface, would correspond to the 

 position of the retina behind the anterior surface of the cornea. The 

 refracting surface would be situated about midway between the 

 posterior surface of the cornea and the anterior surface of the lens. 



The optical axis of the eye is a line drawn through the centres of 

 curvature of the cornea and lens, prolonged backwards to touch the 

 retina between the porus opticus and fovea centralis, and this differs 

 from the visual axis which passes through the nodal point of the 

 reduced eye to the fovea centralis ; this forms an angle of 5 with 

 the optical axis. But for practical purposes the optical axis and the 

 visual axis may be considered to be identical. 



The visual or optical angle (fig. 580) is included between the lines 

 drawn from the borders of any object to the nodal point; if the 

 lines are prolonged backwards they include an equal angle. It has 

 been shown by Helmholtz that the 

 smallest angular distance between 

 two points which can be appreci- 

 ated as two distinct points = 50 

 seconds, the size of the retinal 

 image being 3'65yu ; this is a little 

 more than the diameter of a cone 



at the fovea Centralis Which = 3/X, FIG. 5SO. Diagram of the optical angle. 



the distance between the centres of 

 two adjacent cones being = 4/m. If the two points are so close 

 together that they subtend a visual angle less than 50 seconds, both 

 images will fall upon one cone, and the two points will therefore 

 appear as one. 



Any object, for example, the arrow A B (fig. 581), may be con- 

 sidered as a series of points from each of which a pencil of light 

 diverges to the eye. Take, for instance, the rays diverging from the 

 tip of the arrow A ; C C represents the curvature of the schematic 

 or reduced eye ; the ray which passes through the centre of the circle 

 of which C C is part is not refracted ; this point is represented as 

 an asterisk in fig. 581 ; it is near the posterior surface of the crystal- 



