MancJicstcr Memoiis, Vol. Iv. (191 1), No. ?il. 13 



With a cylindrical lens, either the major or minor axis 

 of the figure marked zero will just extend across the field 

 of view, and the direction of this axis will coincide with 

 the direction of the axis of the cylinder, which can thus 

 be found. 



The effect of combining two cylindrical lenses with 

 the axes of the cylinders at any angle, can readily be 

 shown and the power of the combination in different 

 planes determined. For example, if a pair of cylindrical 

 lenses of the same power and sign be placed, one above 

 the aperture and the other below, with the axes of the 

 cylinders crossed at right angles, the appearance shown 

 will be that of concentric circles, proving that a combina- 

 tion of this kind is equivalent to a spherical lens of the 

 same power.* 



The dioptriemeter is also useful for showing that the 

 power of a combination of thin lenses in close contact is 

 the algebraical sum of the powers of the separate lenses. 

 In determining the power of a concave lens which is 

 beyond —10 D, or a convex lens beyond +15 D, the 

 method of combination may be applied. This is effected 

 by placing the lens in its proper position according to sign 

 and combining it with a suitable lens. The power of 

 the combination having been read off, the power of the 

 lens under test is found by subtraction after finding the 

 power of the auxiliary lens. The value so obtained is 

 but approximate, for the thickness of the lens and its 

 distance from the aperture prevent accuracy. 



When a convex lens of 14 to 15 D is in position, the 

 accuracy with which the scale can be read is much 

 greater owing to the magnification. Readings can then 

 easily be taken to i/s D or about r4%. The focal length 



* For other examples see S. 1'. Thompson " On Ohliquely crossed 

 Cylindrical Lenses," Phil. Mag:, vol. 49, p. 316, 1900. 



