chap, xxviii.] COMPOUND MICROSCOPE. 365 



By this screw the whole body of the instrument above 

 / is moved up or down on a pillar, and so focussing 

 is effected. 



The magnifying power of such a microscope can 

 be affected in three ways : (1) by different lenses, (2) 

 by different eye-pieces, and (3) by the extent to which 

 the draw-tube D is pulled out of the tube t. In- 

 creased magnification by different lenses is already 

 understood. The eye-piece, it has been seen (page 

 359), magnifies the real image formed by the objective, 

 and this image may be magnified more or less accord- 

 ing to the power of the eye-piece. The shorter the 

 eye-piece the more does it magnify. A short eye-piece 

 is often called " deep." Great magnification by the 

 eye-piece is objectionable, since any faults caused by 

 the object-glass are also magnified. By increasing the 

 length of the tube the magnifying power is increased. 

 The increased length is effected by pulling out the 

 draw-tube D. Many instruments have a scale marked 

 on the draw tube, so that the distance it is pulled out 

 may be accurately known. Loss of light follows 

 increased length of the tube, since the light is thus 

 distributed over a greater length, and fewer rays will 

 be focussed by the field-glass of the eye-piece. With 

 each microscope two eye-pieces at least are supplied, 

 a long one, one of small magnifying power, and a 

 short one, of higher magnifying power. The objectives 

 are usually numbered or lettered. Thus, in Zeiss's 

 list, A objective magnifies 38 diameters with No. 1 eye- 

 piece, 52 with No. 2 eye-piece, and 71 with No. 3 ; 

 B magnifies by 70 diameters with No. 1 eye-piece, 95 

 with No. 2, and 130 with No. 3 ; D objective magni- 

 fies by 175 diameters with No. 1 eye-piece, 230 with 

 No. 2, and 320 with No. 3. With Zeiss's instru- 

 ment, the student would have an admirable microscope, 

 using lenses A and D and eye-pieces Nos. 1, 2, and 3. 

 Such an instrument (without Abbe's condenser) 



