THE PROJECTION MICROSCOPE 197 



the course clear, and focussing on the screen, through the 

 alum-cell (by sliding the light to and from the condensers) 

 the surface of the lime cylinder, shown by the granulation of 

 its surface. This will be approximately the distance of the 

 lime from the condensers, and when that has been corrected 

 in use, as presently mentioned, a mark can be made on the 

 tray of the jet, to which it can be at once set in future. 

 Placing in position an objective, with its appropriate substage 

 condenser, the lime should also be most carefully centred. 



Most of the substage condensers have their special func- 

 tions as marked. The low r est power is however used for all 

 objectives up to the T 8 objective ; but for the 2^ inch and 1^ 

 inch powers it is inserted in the front end of its fitting, in 

 order that it may be racked up almost close to a large slide, 

 and illuminate with its cone a large field, whereas with the 

 T^ power it is placed in the back end of its fitting, being 

 focussed upon the slide. If the microscope is fitted with 

 the rotating diaphragm-stage, and the small holes are used, 

 always lift the spring-clips from its surface by the milled 

 head, before rotating the stage. The use of an aperture gives 

 a better edge to the disc when no eye-piece is used, but is not 

 otherwise necessary. 



Let us now suppose the first slide placed upon the stage 

 (we are here considering the commencement of an exhibition). 

 Choose an objective for it (with its appropriate sub-stage con- 

 denser) of the loivest power that will show what is required ; 

 bearing in mind also that, where possible, it is best to choose 

 one that w r ell covers all that is wanted, without showing more 

 than is necessary. This being screwed into the fitting and 

 pushed into the socket, can now be focussed by the rack ; the 

 screw fine -adjustment is only needed with high powers. 



The illumination has next to be got right. It may 

 probably be but poor ; and this must now be remedied by 

 racking the substage condenser backwards or forwards as 

 required. The best position will be almost instantly found, 



