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drawn through a on the principal vertical and b on the nodal vertical will cut 

 the optical axis at the posterior principal focus, and vice versa. 



If a luminous point outside the anterior principal focus is considered, 

 it is obvious that rays from it will be so refracted when they enter the convex 

 surface that they will become converging and will ultimately meet again in 

 a point or focus. Two such points form conjugate foci, figure 454. If the 

 anterior focus of a conjugate is moved away from the anterior principal focus, 

 then the posterior conjugate will move toward the posterior principal focus, 

 and the converse. If one conjugate is known, the other can be found as 



FIG. 454. Diagram to Show the Relations of Conjugate Foci, cd, Refracting surface; 

 AB and ba, conjugate foci; o, nodal point; F", posterior principal focus. 



follows: From a point in the plane of the known conjugate, but outside the 

 principal axis, draw two rays, one perpendicular to the refracting surface 

 which will pass through the nodal point, the other parallel to the principal 

 axis. The latter will be refracted through the posterior principal focus and 

 when prolonged will meet the first ray in the plane of the second conjugate, 

 figure 454, a, This relationship between conjugate foci is played upon in the 

 focusing of a camera. 



It is quite obvious that the eye, even considering only the three surfaces 

 above indicated, is a much more complicated optical apparatus than the one 

 described in the figure. It is, however, possible to reduce the refractive 

 surfaces and media to a simpler form when the refractive indices of the dif- 

 ferent media and the curvature of each surface are know r n. All of these 

 data have been very carefully collected. They are as follows: 



Index of refraction of aqueous and vitreous = J -3365 



Index of refraction of the lens = 1.4371 



Radius of curvature of cornea = 7.829 mm. 



Radius of curvature of anterior surface of lens = 10.0 



Radius of curvature of posterior surface of lens = 6.0 



Distance between anterior surface of cornea and anterior sur- 

 face of lens = 3.6 



Distance between anterior surface of cornea and posterior 



surface of lens = 7.2 mm. 



With these data it has been found comparatively easy by mathematical 

 calculation to reduce the different refractive surfaces of the different curva- 



mm. 

 mm. 



mm, 



