PROPERTIES OF LENSES. 15 



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 

 alienations, increase the angular aperture, and at the 

 same time reduce the number of leuses. 



For an example of tho ingenuity displayed in over- 

 coming these difficulties, we refer the reader to the 

 description of Mr. WVuham'snew objective published 

 in the Proceedings of l/ic 'lloijtd 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 middlo triple 

 in which a single concave lens of flint, three times 

 the focal length of the front, corrects all the others. 



In this combination only five lenses are used, and 

 the errors arising from the sixteen surfaces of glass 

 in tho older forms reduced to ten. 



1. A good objective should possess the Standard 

 screw, adopted by the various societies, and now 

 fixed by the best makers to their leuses. 



2. Its definition should be clear, tho field flat, 

 objects at tho odgo of tho field should be as free 



