14 PROPERTIES OF LENSES. 



such perfection that it seems impossible to improve 

 them. They are designated from four inches up to one 

 fiftieth of an inch, according to their magnifying 

 power : for example, a one inch objective is supposed 

 to magnify as much as a single lens of one inch focus, 

 although its own focal length may be different. 



This rule is only approximately correct, as glasses 

 of the same designation by different makers vary in 

 magnifying power. 



Low powers up to two inches generally consist of 

 a single combination, but as it has been found im- 

 practicable to construct objectives of high power free 

 from chromatic and spherical aberrations in this way, 

 all good makers have adopted the plan of correcting 

 one combination by another, so that objectives from 

 two inch to one inch generally consist of two, while 

 those of quarter inch and upwards generally consist 

 of three combinations, formed perhaps of as many as 

 eight different lenses. 



Various devices have been adopted to lessen the 

 aberrations, increase the angular aperture, and at the 

 same time reduce the number of lenses. For an ex- 

 ample of the ingenuity displayed in overcoming these 

 difficulties, we refer the reader to the description of 

 Mr. Wenham's new objective published in the Pro- 

 ceedings of the Royal Society, vol. xxi. p. Ill, and 

 content ourselves with mentioning that it consists of 

 a single front of the usual form, a single plano-convex 

 back whose focus is four and one half times that of 

 the front, and a middle triple in which a single con- 

 cave lens of flint, three times the focal length of the 



