480 MANIPULATION. 



" The adjustment which is given to object-glasses of high 

 magnifying power, and transmitting large angular pencils of 

 light, is for the purpose of compensating the aberration result- 

 ing from the various states in which an object may be placed. 

 To eflfect this there are two lines on the external part of the 

 object-glass; against the upper line is engraved 

 uncovered, and against the lower, covered; there 

 is also a small square piece of brass, or tongue, 

 screwed into a morticed hole, with a single line 

 upon it, as shown in fig. 263. Immediately above 

 the lines is a projecting milled edge, which may 

 Fig. 263. Hjg moved independently of the other part of the 

 object-glass, giving motion to the part marked Uncovered and 

 Covered; so that either of the lines may be made to coincide with 

 that on the tongue. This motion has the effect of separating or 

 bringing nearer together the lenses which compose the object- 

 glass. When the line against which uncovered is engraved coin- 

 cides with that on the tongue, the adjustment is perfect for view- 

 ing an opaque or uncovered object; but when the line against 

 which covered is marked coincides with that on the tongue, 

 the object-glass is in adjustment for viewing an object covered 

 with glass or talc one-hundredth of an inch thick. If the glass 

 or talc is less than one-hundredth of an inch thick, then the 

 mark on the tongue should be between the marks Covered and 

 Uncovered; and if it exceed one-hundredth, then the mark on 

 the tongue should be without the mark against which covered 

 is engraved. This adjustment must 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 adjustment of the microscope. In this pro- 

 cess the milled edge of the object-glass will be employed to 

 adjust for character of definition, and the fine screw movement 

 of the microscope for correct focus." 



The earlier object-glasses of high power, made by Messrs. 

 Smith and Beck, have the tube of their front lens moveable, 

 and furnished with a screw collar, the circumference of which 

 is engraved with ten divisions, numbered from to 9; this, 

 and the graduation on the milled head for slow motion, give 



