758 



THE EYE AND VISION. 



[CH. LV. 



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. 578) is included 

 between the lines drawn from the borders of any object to the 

 nodal point ; if the lines be prolonged backwards they include 

 an equal angle. It has been shown by Helmholtz that the 

 smallest angular distance between two points which can be appre- 

 ciated = 50 seconds, the size of the retinal image being 3'65/u.; 



Fig. 579. Diagram of the course of the rays of light, to show how an image is formed 

 upon retina. The surface C C should be supposed to represent the ideal curvature. 



this practically corresponds to the diameter of the cones at the 

 fovea centralis which = ^p., the distance between the centres 

 of two adjacent cones being = 4/u,. 



Any object, for example, the arrow A B (fig. 579), may be 

 considered 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. 579; it is near 

 the posterior surface of the crystalline lens ; the ray A C, 

 which is parallel to the optic axis 0', is refracted through the 

 principal posterior focus P, and cuts the first ray at the point A' 

 on the retina. All the other rays from A meet at the same 

 point. Similarly the other end of the arrow B is focussed at B', 

 and rays from all other points have corresponding focuses. 



It will thus be seen that an inverted image of external objects 



