12 Gee and Adamson, Dioptriemeters. 



pin-hole disc. Such a lens will make no appreciable 

 difference in the scale reading. 



An important use of the instrument is for the testing 

 of cylindrical and sphero-cylindrical lenses. When such 

 lenses are used the circles appear distorted into curves 

 somewhat resembling ellipses. With a cylindrical lens, 

 the polar equation is of the form : 



I cos'-^ sin-'9 

 r a b 



where a and b arc the semi-axes. 



Fig. 7. — Scale of Dioplriemeter seen through a convex 

 cylindrical lens. 



The equation to the ellipse with the same semi-axes is : 



I cos^r^ .sin-/J 



?•- ~ d- h- 



The power of the lens in any plane, or meridian, can 

 be determined by observing the scale number of the 

 elliptical type of figure which intersects the boundary of 

 the field of view in that plane, or, better, by rotating the 

 lens until the selected plane intersects the wire that is 

 used for taking accurate readings. 



