36 THE MICROSCOPIST. 



A variety of forms have been given to the mechanism of the 

 compound microscope, many of which are very good, while 

 others are exceedingly objectionable. Suffice it to say respect- 

 ing them, that steadiness, or freedom from vibration, and 

 particularly freedom from any vibrations which are not equally 

 communicated to the object under examination and to the 

 lenses by which it is viewed, is a point of the utmost conse- 

 quence. A microscope body containing the lenses, screwed by 

 its lower extremity to a horizontal arm, is the worst form con- 

 ceivable. 



The compound microscope consists of three parts the opti- 

 cal part, containing the object-glasses and eye-piece; the stage 

 for holding the object ; and the illuminating apparatus, which 

 is either a mirror below the stage for transparent objects, or 

 an illuminating lens for those which are opaque. Whatever 

 form may be given to the mechanical arrangement, the parts 

 alluded to are found in all, and the principles of their manage- 

 ment are the same. 



The most celebrated artists in the manufacture of these 

 instruments are Powell and Lealand; Ross; and Smith and 

 Beck, of London. A microscope from the latter firm is repre- 

 sented in the opposite cut. 



The body slides by a rack and pinion, moved by the milled 

 head, a, on a strong dovetailed bar ; and has also a slow mo- 

 tion for delicate adjustment of focus, given by the milled head 

 b. It is furnished with a sliding tube, c, for varying its length ; 

 and with three sliding Huygenian eye-pieces, tZ, d f , d", of 

 successive powers. 



The erecting glasses, ?/, are to be screwed, when employed, 

 into the other end of the sliding tube. They rectify the 

 image, which is inverted when seen in the usual way. Their 

 chief advantage is in microscopic dissection. 



The stage has two steady rackwork motions, at right angles 



