184 



Bacteriological investigations absolutely require 

 an oil immersion objective in which the T ^ inch of 

 1.32 N. A. is generally employed. 



Botanical work necessitates a 2 inch or 3 inch in 

 addition to the regular outfit. 



Objectives of Wide Aperture. It will be 

 noted that objectives of the lowest price and lowest 

 in the scale of efficiency have been recommended 

 as ample for ordinary use, but it is well to bear in 

 mind or study the advantage, which is obtained by 

 objectives of larger angular aperture. These 

 advantages are absolute and unquestionable, but 

 whether commensurate with the additional pecu- 

 niary outlay, must be left mostly to the judgment 

 of the purchaser. That he may be somewhat 

 guided, we may say that the selection of higher or 

 highest grade objectives is not by any means 

 exceptional, but general, and would undoubtedly 

 be more common but for the barrier of expense. 



In these days of competition, prices alone are 

 too often made the object of inducement, without 

 any reference to quality. Be distrustful of all 

 such objectives and if contemplating their pur- 

 chase, always reserve the right of having them 

 examined by an expert. Have a distrust especially 

 of all " nameless " objectives. It is safe to assume 

 that if the maker cannot attach his name he is 

 doubtful of their quality. 



