THE CORRECTION OF THE OBJECTIVE. 



65 



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 correction for colour. 



The mechanism for applying these principles to the 

 correction of an object-glass under the various circum- 

 stances, is represented in fig. 39, 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 &, for the 

 purpose of varying the dis- 

 tance, according to the thick- 

 ness 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 together 

 when adjusted. An aperture 

 is made in the tube a, within 

 which is seen a mark engraved 



on 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 hun- 

 dredth of an inch thick, and such glass can be readily 

 supplied. This adjustment should be tested experi- 

 mentally 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 adjustment 

 of the microscope. In this process the milled edge of 



FIG. 39. 



