68 PRACTICAL TREATISE ON THE MICROSCOPE. 



optical apparatus, both of which are very much more com- 

 plicated than they are in the former instrvunents. The stand 

 is made up of the compound body (or tube for carrying the 

 optical apparatus), and the stage with the supports and ad- 

 justments for each; whilst the optical part consists of the 

 object-glasses or magnifying powers, the eye-pieces, and the 

 mirror. It Httle matters what the shape or size of the instru- 

 ment may be ; for whatever plan is adopted, or in whatever 

 country it may be made, the parts above described are strictly 

 essential, and must be present in each. The compound body 

 is generally a tube of brass, from eight to ten inches in length, 

 and from an inch to an inch and a half in diameter ; to its 

 upper end the eye-pieces are adapted, to its lower the object- 

 glasses; as these latter are of different magnifying powers, 

 and as no two objects under examination are of the same 

 thickness, it is highly requisite that there should be some 

 mode of focal adjustment applicable to every condition. This 

 is effected in two ways, one of which is termed the coarse, 

 the other the fine adjustment; the first is generally accom- 

 phshed by rack and pinion, by which the whole of the tube 

 carrying the eye-piece and object-glass is made to approach 

 or recede from the object by turning a large milled head 

 connected with the pinion ; whilst in the second or fine ad- 

 justment, the object-glass only is moved, and that by means 

 of a very dehcate screw, acting either on the long end of a 

 lever or in some of the modes Ijereafter to be noticed, 

 whereby the same result is obtained. In the best constructed 

 stands, the entire compound body containing the magnifiers is 

 moved up and down by the coarse adjustment, but in many 

 of the older microscopes, as represented by fig. 21, there were 

 two or more tubes to make up the compound body. When 

 this was the case, the outer tube was firmly attached to the 

 other part of the stand, and formed the guide for the inner 

 one carrying the optical apparatus to sUde through; under 

 these circumstances the rack-work was placed in the com- 

 pound body itself; but much greater stability is ensured by 

 the adoption of the former method. In some instruments 

 there is a tube connected with the compound body, capable 



