Dry, Adjustable Objectives. Adjustable objec- 

 tives, or objectives with collar correction are those in 

 which there is a mechanical provision to vary the 

 distance between the lenses, in order to make with 

 facility, proper correction for spherical aberration. 

 They involve a high degree of mechanical perfection 

 and are therefore more expensive than objectives with 

 fixed mountings. They may, however, be recommended 

 to microscopists who have acquired some experience 

 in handling objectives and even to beginners who will 

 use judgment in their use, as they certainly give excel- 

 lent results and quick means for obtaining the utmost 

 limit of efficiency in objectives, a fact best appreciated 

 by those who are expert in the use of them. In these 

 objectives, as at present constructed by the best 

 makers, a milled collar is provided, which when 

 rotated imparts a rectilinear motion to an interior tube 

 carrying the posterior system of the objectives, thus 

 varying the distance between them and the front 

 system which remains stationary. The screw collar is 

 graduated in such a manner that the figures indicate 

 the correct point for the proper thickness of cover 

 glass; thus 10 indicates proper correction for a cover 

 of o.io mm., 1 6 for 0.16 mm., and so on. 



When set for thick covers, the lenses are closest 

 together and the adjustment is said to be closed : when 

 for thin covers, the lenses are farthest apart and the 

 adjustment is open. 



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