THE SENSE OF SIGHT 



689 



efractive indices of the first and last media. Planes passing through the 



focal points vertically to the axis are known as focal planes. 



From these properties of the cardinal points the position of an image in 



the last medium of a luminous point in the first may be determined, and the 



bourse of a refracted ray in the last medium be constructed if its direction in 



'the first be given according to the following rules: 



fi. To find the image in the last medium of a luminous point in the first : Let 

 A (Fig. 293) be this given point. Draw A B parallel to the axis until it 

 meets the second principal plane in ; then B F 2 will be this ray after 

 refraction. Draw a second ray from A to the first nodal point; then 

 draw another ray, D E, from the second nodal point parallel to A C. 

 This will be the refracted ray in the last medium. Where the two re- 

 fracted rays, BF 2 and D , intersect, the image of A will be A r * 



' 2 To find the refracted ray in the last medium of a given ray in the first 

 medium: Let A B (Fig. 294) be the given ray. Continue this ray until 



or A GIVEt 



it meets the first principal plane in C. Draw C D 



Now assume any point, such as E, in the given ray, and find its 



E, by the Rule i. Then D E, becomes the course of the refract 



The'Schematic Eye.-Accepting the system of cardinal points, Listing, 

 Bonders, and v. Helmholtz have constructed "schematic" eyes to be sub- 

 ctitntpH for the refracting system of the natural eye. 



For thfs purpose U is necessary to make use of the various estunates of 

 ^he indices of refraction of the different media, of the radii of curvatures 

 3 the different refracting surfaces, and of the distances separating them to 

 deduce an Ivlge eye ^a basis for calculation. The most widely accepted 

 attempts thatofv. Helmholtz. The data he assumed are as follows: The 

 re Se index of air-i; of the cornea and aqueous humor, 1.3365; of he 



the point A is infinite,, 



A, owhere all paraUe, to the n 



meet. 



44 



