DESCRIPTION OF TUB ACHROMATIC MICROSCOPE. 03 



viewing opaque objects by silver cups, where a is the 

 concave speculum, ovLeiberkuhn. 



This stand differs from the other in the following par- 

 ticulars. The pillar is composed of a couple of tubes, 

 the one sliding within the other. The triple-milled 

 ring, q, screws upon the top of the outer one, and 

 serves to fix them together at any required elevation. 

 The base, u, u, u, consists of one solid piece of metal, 

 the form of which is that which several years' experience 

 has proved to be the best. Dr. Goring, in Chap. VII., 

 clearly demonstrates that a tripod bearing is indispen- 

 sable to insure firmness. In his instrument, however, 

 the base, instead of being solid, is composed of three 

 pieces (see plate, fig. 7)> the objection to which is, that 

 notwithstanding a screw is used to counteract their 

 springing, the stand is yet liable to rock. I have lately 

 made some of his instruments with a solid base, similar 

 to my own, which are decidedly preferable. 



My tripod form has been objected to, although the 

 solid base is now adopted in all the best instruments. 

 It has been said to occupy more room than those having 

 folding legs, and, therefore, to require a larger case ; and 

 again, that it should be equilateral, having its three feet 

 of equal length. The answer to this is, — first, that no 

 folding stand can be rendered perfectly firm, and that 

 there is some trouble and loss of time in arranging it for 

 use j and should it, through any hurry or mistake, be 

 set up without properly unfolding the feet, the instrument 

 is very liable to be injured. And, secondly, the base 



