320 



ACCESSOEY APPARATUS 



The objective most suitable is a nyths of '65 N.A. ; but a Jth of -71 

 N.A., or a ^-rd of '65 N".A. will do equally well, as the whole of the 

 back lens of the objective should be visible through the analysing 

 ' Nicol ; ' the back lens of the objective must not be too large, thus a 

 ^ inch of '65 N.A. would not do so well. The analysing prism may 

 be placed either where it is in the drawing or above the eye-piece. 

 Practically it works very well above the 

 objective, which is the position it occupies 

 in ' ordinary microscopical outfits.' 



For the draw-tube a 2 -inch objective 

 and a B or C eye-piece will answer 

 admirably. 



FIG. 264. Swift's illuminating and polarising 

 apparatus. 



FIG. 265. In this diagram P 

 is the polarising prism in 

 the sub-stage, C sub-stage 

 condenser. On the stage 

 M mineral. On nose-piece 

 O l objective tfeths '64 N.A. ; 

 A analysing prism. In the 

 draw-tube, O a objective 2 

 or 3 in. H, Huyghenian 

 eye-piece. 



For setting up the instrument it is better, before screwing the 

 objective in the end of the draw tube, to centre the light in the 

 usual manner, the ' Mcols ' being turned so as to give a light field. 

 Next fix the objective in the draw-tube, open the sub-stage con- 

 denser to full aperture, and put the mineral on the stage. Rack 



