104 APPENDAGES TO THE MICROSCOPE. 



by the focal adjustment into contact with the surface of the glass, 

 it stamps on this a minute circle enclosing the object. 



75. Stage- Movement. — The general arrangement of the Mecha- 

 nical Stage now usually adapted to all high-class Microscopes has 

 been already explained (§§ 53-55) ; and though the details are 

 differently constructed by the several makers, yet the general prin- 

 ciple is that a lateral or horizontal movement is given to the Object 

 platform by one milled-head, and a front-to-back or vertical move- 

 ment (the Microscope being supposed to be placed in an inclined 

 position) by another. — The Stage may be so constructed, however, 

 that motion shall be given to the object-platform by means of a 

 Lever acting upon it in any required direction ; this being accom- 

 plished by making the object-platform slide laterally on an inter- 

 mediate plate, and by making the latter slide vertically upon the 

 fixed stage-plate which forms the basis of the whole ; each pair of 

 plates being connected by dovetailed slides and grooves. Thus 

 the Object-platform may be readily made to traverse, not merely 

 horizontally or vertically, but, by the simultaneous sliding of both 

 plates, in any intermediate direction. This is especially conve- 

 nient in following the movements of Animalcules, &c. , for which 

 purpose this lever-stage is to be preferred to the ordinary form : its 

 use being attended with this particular facility, that, as the motion 

 of the hand is reversed by the lever, so that the object moves in 

 the opposite direction, whilst the motion of the object is again re- 

 versed to the eye by the Microscope, the image moves in the same 

 direction as the hand ; and thus, with a little practice, even the 

 most rapid swimmer may be kept within the field by the dexterous 

 management of the lever. For general purposes, however, the 

 ordinary Mechanical Stage will be found most convenient ; whilst 

 for following Animalcules, &c, several of the simpler arrangements 

 described in the preceding Chapter answer extremely well. 



76. Object- Finder. — The Mechanical Stage admits of a simple 

 addition, which very much facilitates the ' finding ' of objects 

 mounted in slides, which are so minute as not to be distinguishable 

 by the naked eye ; such, for example, as the particular forms 

 that present themselves in Diatomaceous deposits. This ' finder ' 

 consists of two graduated Scales, one of them vertical, attached 

 to the fixed stage-plate, and the other horizontal, attached to an 

 arm carried by the intermediate plate ; the first of these scales 

 enables the observer to 'set' the vertically-sliding plate to any 

 determinate position in relation to the fixed plate, while the 

 second gives him the like power of setting the horizontally-sliding 

 plate by the intermediate. In order to make use of these Scales, 

 it is of course necessary that the sliding and rotating platform on 

 which the object immediately rests, should be always brought into 

 one constant position upon the traversing plates beneath ; this is 

 accomplished by means of a pair of stops, against which it should 



