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TEXT-BOOK OF PHYSIOLOGY. 



spending principal and nodal points from each other is equal to the differ- 

 ences between the two focal distances. Also the distance of the two 

 principal points from each other is equal to the distance of the two nodal 

 points from each other. Finally, the focal distances are proportional to the 

 refractive 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 



FIG. 310 DIAGRAM TO FIND THE IMAGE IN LAST MEDIUM OF A LUMINOUS POINT IN 



THE FIRST. 



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



the first be given according to the following rules : 



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

 A (Fig. 310) be this given point. Draw A B parallel to the axis until it 

 meets the second principal plane in B ; 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 E, intersect, the image of A will be A r l 



FIG. 311. DIAGRAM TO FIND THE REFRACTED RAY IN THE LAST MEDIUM OF A GIVEN 

 RAY IN THE FIRST MEDIUM. 



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

 medium: Let A B (Fig. 311) be the given ray. Continue this ray until 

 it meets the first principal plane in C. Draw C D parallel to the axis. 

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

 by the Rule i. Then D E l becomes the course of the refracted ray. 



1 If the point A is infinitely far from the eye, all the rays striking the eye will be parallel to 

 each other. The nodal ray must therefore be drawn, and the point where this nodal ray meets 

 the second focal plane will be the image of A , or A t where all rays parallel to the nodal ray will 

 meet. 



