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TEXT-BOOK OF PHYSIOLOGY. 



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 pro- 

 portional 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. 



FIG. 285. DIAGRAM TO FIND THE IMAGE IN LAST MEDIUM OF A LUMINOUS POINT 



IN THE FIRST. 



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 course of a refracted ray in the last medium be 

 constructed if its direction in the first be given according to the fol- 

 lowing rules : 



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

 first: Let A (Fig. 285) 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 



FIG. 286. DIAGRAM TO FIND THE REFRACTED RAY IN THE LAST MEDIUM OF A 

 GIVEN RAY IN THE FIRST MEDIUM. 



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 refracted rays, BF 2 and 

 D E, intersect, the image of A will be A t .* 



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

 first medium : Let A B (Fig. 286) be the given ray. Continue this 



* 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 = A v 

 where all rays parallel to the nodal ray will meet. 



