THE COMPOUND MICROSCOPE. 9 



diaphragm serves for the reception of a dull or coloured glass 

 disk. The mirror is plane on one side (for low power work) 

 concave on the other. When using the condenser the plane side 

 of the mirror should be used, and the amount of light regulated by 

 the iris-diaphragm. For low powers this should have its opening 

 contracted, for high powers widened ; if an oil immersion lens is 

 used with stained preparations (particularly with bacteria), the 

 iris-diaphragm can be left wide open. 



The lenses of Leitz are, for their price, of exceptionally high 

 quality. Any eye-pieces or objectives can be purchased, but 

 for initial use perhaps the best combination is objectives 3 

 ( = approximately f inch of English classification) and 6 ( = J inch) 

 or 7 ( = | inch approximately) ; and eye-pieces Nos. 0, II. and IV. 

 Without the use of the draw-tube this then gives the following 

 range of magnification : objective 3 and eye-piece = 46 dia- 

 meters ; 3 and II., 70 ; 3 and IV., 105 ; 6 and 0, 210 ; 6 and II. , 

 330 ; 6 and IV., 480 ; or 7 and 0, 270 ; 7 and II., 440 ; 7 and IV., 

 625. These objectives work best with the same tube length as 

 in the instrument previously described, and the use of the draw- 

 tube for further magnification is subject to the same criticisms. 

 The double nose-piece of this firm is also of exceptionally good 

 construction. Various supplementary appliances, e.g., for drawing, 

 polarisation, etc., or for still higher magnification, can be added 

 at any time, as finances allow r . 



To place the Microscope for Work. The microscope can be 

 used either by solar or artificial light, but where possible the 

 former is to be preferred. The microscope is placed opposite to 

 a window, and at a distance of a yard and a half or so away from 

 it, and in such a position that the direct sun's rays do not fall 

 upon it. If the direct sun's rays fall on the window an admirable 

 screen, producing perfect illumination, is provided by a sheet of 

 glazed linen between microscope and window ; much the same 

 kind of light, in fact, as is produced in the ideal illumination from 

 a white cloud. If desired the microscope can be used left hand 

 to the light, but front position is preferable. While looking down 

 through the microscope, we adjust with the fingers the slope of 

 the mirror till the field of view appears bright and equally lighted. 

 If there are initial difficulties, remove the objective, or twist the 

 nose-piece crosswise, and adjust the mirror with eye-piece only. 

 If the mirror arm is jointed, see that you do not twist the centre 

 of the mirror out of the line of the axis of the instrument. 



