'GENERAL RULES FOR THE CONSTRUCTION OF STANDS. 117 



orients has, of course, its advantages and disadvantages, and their 

 relative estimation is, to a great extent, a matter of individual 

 judgment. Mohl expresses himself decidedly against any move- 

 ment of the stage, as tending to the development of general 

 unsteadiness, and because the screw-micrometer, which is in- 

 dispensable in his opinion, can only be applied efficiently to a 

 fixed stage. The first reason is at any rate worthy of notice; 

 the last is less evident on account of the slight use made of the 

 screw-micrometer. On the other hand, with the fixed body-tube 

 is combined the advantage that the head can always be kept at 

 the same height when different objectives are employed, while in 

 the opposite case its position depends upon the magnifying power. 

 The differences of height which the focusing involves are incon- 

 venient only with the low-power objectives, and may be overcome 

 by suitable shortening of the body-tube. The more important 

 reasons are therefore in favour of the perfect immovability of 

 the stage in the direction of the optic axis. 



The coarse adjustment is effected either by a pinion which 

 may be applied in different ways, or by sliding the body-tube 

 in a slightly elastic socket by the hand. The former method is 

 now employed in most of the larger instruments, the latter in the 

 majority of small Microscopes, both German and French. The 

 socket surpasses the pinion in simplicity and smoothness of 

 motion; but by long use the surfaces of contact become worn, 

 and dust and oxidation injure them. The tube is therefore apt 

 to slide either too easily or the reverse, unless its fitting is main- 

 tained in proper condition. The inconvenience of its sliding by 

 its own weight can be avoided by slight pressure of the socket. 



The principle of lever-movement, which is in most English 

 instruments employed for different purposes, has been turned to 

 account in the large Microscopes of Ploessl for the coarse adjust- 

 ment. The arrangement in question is recommended as "a 

 substantial and convenient construction." 



The object-stage should be sufficiently large (at least 2J to 3 

 inches in diameter), plane, and as firm as possible. Projecting 

 screw-heads, fixed spring-clips, &c., are undesirable ; they interfere 

 with the application and free movement of large glass plates 

 in the plane of adjustment. The stage may be circular or 

 square ; its upper surface may be black or not ; the objectionable 

 glare of light, which is often seen in the Microscope when low 



