Curvature Method of Teaching Optics. 477 



and the radius of the zone or decentration in cm. The more 

 exact equation is also a valuable introduction to Von SeidePs 



Fiff. 4. 



method of deviations. From 

 tan a { 



u 



Cylindrical Lens. 



the equation m = ^ we have 

 Helmholtz's expression for the 



or 



immediately m = . 



J tan a 2 



magnification. 



Combinations of Thin Lenses in contact. — It is obvious from 

 the summation of the thicknesses that the resultant conver- 

 gence F = XFj for any n amber of thin lenses in contact, and 

 that in the case of cylindrical or sphero-cylindrical lenses, 

 the ] towers in any meridian may be added. It has been 

 already shown that the convergence of a cylindrical lens in a 

 meridian making an angle of 6 with the axis is 



F sin* = 2 H -cos 20;. 



Hence it' we have any number of cylindrical lenses in contact 

 of convergences h\, F 2 . A:c, and whose axes make angles «j, 

 ■., with a reference meridian, we have 



V = £2F 1 -£2F 1 cos2(0-a 1 ) 



