712 



HANDBOOK OF PHYSIOLOGY. 



curvature of the cornea and lens, prolonged backward 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 



Fig. 421. Diagram of the optical angle. 



the fovea centralis; this forms an angle of 5 with the optical axis. 

 By some the optical axis and the visual axis are considered to be iden- 

 tical. The visual or optical angle is included between the lines drawn 

 from the borders of any object to the nodal point; if the lines be pro- 



Fig. 421 A. Diagram of the method of the formation of an inverted image exactly focussed 

 upon the retina. The dotted line is the ideal surface of curvature. 



longed backward they include an equal angle. It has been shown by 

 Helmholtz that the smallest angular distance between two points which 

 can be appreciated = 50 seconds, the size of the retinal image being 

 3.65/e; this practically corresponds to the diameter of the cones at the 



Fig. 422. Diagram of the course of a ray of light, to show how a blurred or indistinct image 

 is formed if the object be not exactly focussed upon retina. The surface C C should be sup- 

 posed to represent the ideal curvature. The nodal point should be nearer the posterior surface 

 of lens as in fig. 421 A. 



fovea centralis which = 3,u, the distance between the centres of two ad- 

 jacent -cones being = 4,a. 



The image of an object, then, is thus formed upon the retina. An 



