ADVANCED EXPERIMENTAL PHYSIOLOGY 



365 



similarly very near, being distant only about -4 mm. from each other. 

 We therefore take a "mean" nodal point and a "mean" principal 

 point and again reduce the optical conditions to those of a simple 



FIG. 240. Diagrams to illustrate refraction. 



optical system, consisting of one (ideal) refractive surface. In such 

 a " reduced eye " the cardinal points are as follows : 



Principal point. 2-3448 mm. behind the anterior surface of the 



cornea, in the aqueous humour. 

 Nodal point. -4764 mm. in front of the posterior surface of 



the lens. 

 Posterior principal focus. 22-647 mm. behind anterior surface of 



the cornea. 

 Anterior principal focus. 12-8326 mm. in front of the anterior 



surface of cornea. 

 Radius of curvature of ideal surface, 5-1248 mm. 



With these data we are now able to understand the formation of the 

 image on the retina, and are able to calculate the size of the retinal 

 image of an object. 



