ADVANCED EXPERIMENTAL PHYSIOLOGY 229 



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

 the lens. 



FIG. 209. Diagrams to illustrate refraction. 



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



the cornea. 

 Anterior principal focus. 12 -8 326 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. 



A ray passing through the nodal point K (Fig. 240) will not undergo 

 refraction, and therefore will indicate the position of the image of any 

 external point upon the retina. It follows also that the size of the actual 

 image may be calculated if we know AB (the size of the external 

 image), dK its distance from the nodal point. For 



ab_AB 

 Kr~ dK' 



