MOUNTING OF OBJECTIVES. 129 



MOUNTING OF OBJECTIVES. 



Having glanced briefly at the optical portion, a few 

 words in reference to the mechanism of the instrument 

 may not be out of place. 



Adjustable glasses are provided with movable or 

 stationary fronts, that is to say, in the process of re- 

 volving the correction collar the front lens also revolves, 

 or remains stationary, as the case may be. The station- 

 ary front being the mobt expensive mounting, it is gen- 

 erally adopted in first-class American or London glasses 

 of short focal distance. 



In the use of objectives of tolerably long focal dis- 

 tance, the necessity for the stationary front is not so 

 apparent, and some first-class makers adopt either form 

 of mounting for such objectives. It behooves the buyer 

 to keep in mind this difference in the cost of the two 

 mountings, for of two glasses, both equal in optical 

 performance, the one adjusting with stationary front 

 ought to be the most costly. 



Generally, the mechanism of the collar adjustment 

 should be first-class. There should be no "slip," "back- 

 lash," or " dead-point;" the collar should rotate with a 

 certain firmness of action, and yet run as " smooth as 

 oil;" there should be no undue rubbing or grating, nor 

 "hitch or hindrance" of any kind. With a one-half 

 inch, or a four-tenths, a slight " slip" or " back-lash" 

 need not defeat an otherwise satisfactory objective; 

 but, on the other hand, nothing of this sort can be 

 allowed in the higher and first-class objectives. If the 



9 Microscopy. 



