CONSTRUCTION OF THE MICROSCOPE. 



microscope require to be set in their respective tubes, sc 

 that their several axes shall be directed in the same 

 straight line with the greatest mathematical precision. 

 This is what is called centering the lenses, and it is a 

 process which, in the case of microscopes, demands 

 great skill on the part of .the manufacturer. The slightest 

 deviation from true centering would 

 cause the images produced by the 

 lenses to be laterally displaced, one 

 being thrown more or less to the right, 

 and the other to the left, or one up- 

 wards and the other downwards ; and 

 even though the aberrations should be 

 perfectly effaced, the superposition of 

 such displaced images would effectually 

 destroy the efficiency of the instru- 

 ment. It should also be so accurate, 

 that the optical axis of the instrument 

 should not be in the least altered by 

 movement in a vertical direction ; so 

 that, if an object be brought into 

 the centre of the field with a low 

 power, and a higher power be then 

 substituted, it should be found in 

 the centre of its field, notwithstanding 

 the great alteration in focus. 



Fig. 31 represents the body of one 

 of Mr. Ross's compound microscopes 

 with the triple object-glass, where o is 

 an object; and above it is seen the 

 triple achromatic object-glass, in con- 

 nection with the eye-piece e e, ff the 

 plano-convex lens ; e e being the eye- 

 glass, and // the field-glass, and be- 

 tween them, at Ib, a dark spot or dia- 

 phragm. The course of the light is 

 shown by three rays drawn from the 

 centre, and three from each end of the 

 object o; these rays, if not prevented by the lens //, or 

 the diaphragm at b b, would form an image at a a; but as 

 they meet with the lens / f in their passage, they are 



