75 



between the two first systems (anterior and middle) 

 is attained by means of a rectilinear motion to the 

 middle and posterior system and stationary anterior 

 system, while in the second the conditions are 

 reversed ; the two posterior systems remain station- 

 ary while the front is adjusted. Both are arranged 

 with graduations upon the screw-collar and have an 

 index. 



In the objectives of the Bausch & Lomb Optical 

 Co., the graduations range from 10 to 35, and as pre- 

 viously stated these numbers indicate the proper 

 points of correction for the respective thickness of 

 cover glass. 



In these objectives the open point, /. e., where the 

 objectives are most widely separated, is at 10, thus 

 giving the correction for practically the thinnest cov- 

 ers. As the objective is moved toward the higher 

 numbers the adjustment is closed and gives correction 

 for the thicker covers. In objectives of other manu- 

 facture where this system is not adopted, the figures 

 are arbitrary and have no special significance. While 

 the open and closed point can be learned in these by 

 trial, it should be given by the maker before work is 

 attempted with it. Before the objective is attached 

 the adjustment should be closed, as, if this is neg- 

 lected and the objective has a short working distance, 

 the front lens may come in contact with the cover 

 when it is endeavored to focus on the object. 



Probably the best object for studying the effect of 

 the screw-collar adjustment and acquiring skill in 

 determining its best point is again P. angulatum. 



