145 



fixed mountings. They may, however, be recom- 

 mended to microscopists who have acquired some 

 experience in handling objectives and even to be- 

 ginners who will use judgment in their use, as 

 they certainly give excellent results and quick 

 means for obtaining the utmost limit of efficiency 

 in objectives, a fact best appreciated by those 

 who use them. In the objectives as at present con- 

 structed by the best makers, a milled collar is pro- 

 vided, which in turning imparts a rectilinear 

 motion to an interior tube, carrying the posterior 

 systems of the objectives,* thus varying the dis- 

 tances between them and the front system, which 

 remains stationary. The screw collar is graduated 

 in such a manner that the figures indicate the cor- 

 rect point for the proper thickness of cover, thus 

 10 indicating proper correction for a cover of 

 0.10 mm., 16 for 0.16 mm. and so on. When set at 

 the highest figure, for thick covers, the lenses are 

 in the closest position and the adjustment is said 

 to be closed. When for the thin covers, are farthest 

 apart and is then open. 



In objectives of older construction and in some 

 produced at the present day, the figures are arbi- 

 trary and serve no other purpose than an index for 

 reference. 



Close the adjustment before attaching the objec- 

 tive as its front may otherwise come in contact 



with the cover before the focus is reached. For 

 10 



