THE COMPOUND MICROSCOPE. 7 



pinion work is cut obliquely instead of horizontally. If, how- 

 ever, the beginner " focusses down," he is certain frequently 

 to over-pass the proper focus, especially with the high power ; 

 and, should the objective come into contact with the cover-glass 

 of the micro-preparation, the results when rack- work is used are 

 likely to be much more serious than with the sliding-tube. The 

 experienced student may focus up or down with almost equal 

 safety, and in practice is pretty sure to focus by the downward 

 movement, so that to him the advantage of the rack-work 

 movement is very considerable. It is largely for the purpose of 

 obviating the necessity of focussing with the high power that I 

 advise, in an earlier paragraph, the use of the double nose-piece, 

 and the correction of the length of the objective settings by 

 paper or other collars, so that they focus naturally for the same 

 plane. 



A very excellent medium-priced microscope for higher student 

 work is the Stand II. a of E. Leitz, of Wetzlar. The workman- 

 ship and optical appliances of this firm have a very, and de- 

 servedly, high reputation, and we will select this instrument as 

 typical of those capable of use for all ordinary forms of higher 

 work, and to which additional appliances can at any time be 

 supplied. It is represented l in Fig. 4. In common with foreign 

 microscopes generally,* the old heavy horse-shoe form of foot is 

 retained, stability being obtained by weight rather than by form ; 

 and the jointing of a single vertical pillar as a means of inclining 

 the body might with advantage be abandoned for some other 

 method. This microscope has a coarse adjustment by rack and 

 pinion, instead of by sliding-tube, and an exceptionally good fine 

 adjustment by micrometer screw of delicate construction, situated 

 at the top of the pillar. It has a draw-tube, with millimeter 

 divisions engraved upon it. Under the stage is an illuminating 

 apparatus on the Abbe principle, with condenser and iris- 

 diaphragm fixed together, so that they can be withdrawn for 

 oblique illumination, for which, also, the mirror is capable of 

 lateral movements. This illuminating apparatus is moved on the 

 optical axis of the instrument by means of a lateral screw showing 

 in the figure, so that it can be focussed upon the object in accord- 

 ance with the thickness of the micro-slide ; and in place of it 

 ordinary diaphragms can be readily used. A ring under the iris- 



1 For price, etc., see Introduction. 



