120 



ACCESSORY APPARATUS 



The objective most Miitablc is a 1 4 , [ ths of -(55 X.A. ; but a j-th of 71 

 N.A.. or a J-nl of (')") X.A. will do equally well, as the whole of the 

 liark lens of tlie olijective should be visible through the analysing 

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

 .', inch of -li.") X.A. would not do so well. The analysing prism may 

 lie placed cither where it is in the drawing- or above the eye-piece. 

 I'rart ically it works very well above the 

 objective, which is the position, it occupies 

 in ' ordinarv microscopical outfits.' 



For the draw-tube a 2-inch objective 

 and a 1> or C eye-piece will answer 

 admirably. 



1( ''l. Swill's illiimiiuting and 

 apparatus. 



Fie;. 265. In this diagram P 

 is the polarising prism in 

 the sub-stage, C sub-stage 

 rondenser. On the stage 

 .AI mineral. On nose-piece 

 O l objective ,',,thsT, I N.A.; 

 A iinalysing prism. In the 

 draw-tube, O- objective 2 

 >r ' in. H, Huyghenian 

 eye-piece. 



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

 ' "' the end of the draw-tube, to centre the li-ht in the 

 fficols' being burned so as to give a light field. 

 the objective in the draw tube, open the sub-stege con- 

 aperture, and put the mineral on the stage Rack 



