70 PRACTICAL TREATISE ON THE MICROSCOPE. 



is accomplished by mounting it upon a short piece of tube^ 

 capable of fitting into another tube in the stage-plate; on 

 this tube it turns, and by it the object-plate is also raised 

 above the working parts of the stage-plate itself. 



The stage movements generally extend from half-an-inch 

 to two inches, so that by the sliding up and down of the 

 object-plate, and the distance the same plate is capable of 

 being traversed over by the rack-work, aU parts of an object 

 of considerable size can be brought in succession into the 

 field of view. The different methods of effecting these stage 

 movements will be described with the instruments to which 

 they are severally adapted. To the under side of the stage a 

 number of other pieces of apparatus can be fitted, viz., the 

 diaphragm plate, the achromatic and other condensers, the 

 lower prism of the polarizing apparatus, and the dark stops or 

 wells, all of which wiU hereafter be described. To the object- 

 plate should also be fitted the forceps for holding opaque 

 objects. 



The methods of mounting the compound body and the 

 stage are exceedingly various, the most improved plans are 

 represented in the following figures, and for our present 

 purpose it will be merely necessary to divide them into two 

 classes ; first into those in which the compound body is sup- 

 ported at its lower end, on an arm capable of being moved 

 up or down by a rack and pinion ; and, secondly, into those 

 in which the compound body is supported, either by an arm 

 firmly attached to the back of it, as seen in fig. 21, when it is 

 necessary that the body should be composed of more than one 

 tube, or where a large portion of its length is supported, after 

 the plan of Mr. George Jackson. 



The remaining portion of the stand of the compound micro- 

 scope consists of the foot or basis, and of one or more pillars 

 or supports rising from it, to which the compound body and 

 stage are attached. The foot is generally a stout tripod of 

 brass, cast in one piece, or, for convenience of package, it may 

 be composed of three flat feet, capable of being folded to- 

 gether, as in fig. 21, or as in two of Mr. Powell's instruments, 

 of three longer legs, standing in an inclined position, like 



