CONSTRUCTION OF THE MICROSCOPE. 



43 



finest definition of extremely delicate and minute objects, 

 they should be viewed without a covering ; if it be 

 desirable to immerse them in a fluid, they should be 

 covered with the thinnest possible film of talc, as, from the 

 character of the chromatic aberration, it will be seen that 

 varying the distances of the combinations will not sensibly 

 affect the correction ; though object-lenses may be made 

 to include a given fluid, or solid medium, in their correc- 

 tion for colour. 



" The mechanism for applying these principles to the 

 correction of an object-glass under the various circum- 

 stances,, is represented in fig. 28, 

 where the anterior lens is set in 

 the end of a tube a, which slides 

 on the cylinder b, containing the 

 remainder of the combination ; 

 the tube a, holding the lens nearest 

 the object, may then be moved 

 upon the cylinder b, for the pur- 

 pose of varying the distance, ac- 

 cording to the thickness of the 

 glass covering the object, by 

 turning the screwed ring c, or 

 more simply by sliding the one on 

 the other, and clamping them to- 

 gether when adjusted. An aper- 

 ture is made in the tube a, within 

 which is seen a mark engraved on 



O 



the cylinder ; and on the edge of which are two marks, a 

 longer and a shorter, engraved upon the tube. When the 

 mark on the cylinder coincides with the longer mark on 

 the tube, the adjustment is perfect for an uncovered 

 object; and when the coincidence is with the short mark, 

 the proper distance is obtained to balance the aberrations 

 produced by glass the hundredth of an inch thick, and 

 such glass can be readily supplied. This adjustment should 

 be tested experimentally by moving the milled edge, so as 

 to separate or close together the combinations, and then 

 bringing the object to distinct vision by the screw adjust- 

 ment of the microscope. In this process the milled edge 

 of the object-glass will be employed to adjust for character 



Tig. 28. 



