MANAGEMENT OF THE MICROSCOPE. 



143 



FIG. 112, 



differ, according to the particular plan of the instrument employed: 

 thus, in some it is only the ledge itself that slides along the stage; in 

 others it is a carriage of some kind, whereon the object-slide rests; in 

 others, again, it is the entire platform itself that moves upon a fixed 

 plane beneath. Having guided his object, as nearly as he can do by the 

 unassisted eye, into its proper place, the Microscopist then brings his. 

 light (whether natural or artificial) to bear upon it, by turning the- 

 Mirror in such a direction as to reflect upon its under surface the rays 

 which are received by itself from the sky or the lamp. The concave 

 mirror is that which should always be first employed, the plane being 

 reserved for special purposes; and it should bring the rays to convergence- 

 in or near the plane in which the object lies (Fig. 112). The distance- 

 at which it should be ordinarily set beneath the stage, is that at which it 

 brings parallel rays to a focus; but this distance should be capable of 

 elongation, by the lengthening of the stem to which the mirror is 

 attached, since the rays diverging from a lamp afc a short distance are 

 not so soon brought to a focus. The correct focal adjustment of the- 

 Mirror may be judged by its formation of images of window-bars, chim- 

 neys, etc., upon any semi-transparent medium placed in the plane of the- 

 object. It is only, however, when small objects are being viewed under- 

 high magnifying powers, that such a concentration of the light reflected 

 by the Mirror is either necessary or desirable; for, with large objects 

 seen under low powers, the field would not in this mode be equally 

 illuminated. The diffusion 

 of the light over a larger area 

 may be secured, either bv 1 

 shifting the Mirror so much 

 above or so much below its 

 previous position, that the 

 pencil will fall upon the object 

 whilst still converging, or 

 after it has met and diverged; 

 or, on the other hand, by the 

 interposition of a disk of 

 Ground-glass in the course of 

 the converging pencil, this 

 method which is peculiarly 

 appropriate to lamp-light, be- 

 ing very easily had recourse 

 to, if the diaphragm-plate 

 have had its larger aperture 



fitted to receive SUCh a disk Arrangement of Microscope for Transparent Objects. 



( 98). The eye being now 



applied to the Eye-piece, and the body being ' focussed/ the object is to- 

 be brought into the exact position required by the use of the transversing 

 movement, if the stage be provided with it; if not, by the use of the 

 two hands, one moving the object-slide from side to side, the other 

 pushing the ledge, fork, or holder that carries it, either forwards or 

 backwards as may be required. It is always to be remembered, in 

 making such adjustments by the direct use of the hands, that, owing to 

 the inverting action of the Microscope, the motion to be given to the 

 object, whether lateral or vertical, must be precisely opposed to that 

 which its image seems to require, save when Erectors ( 84, 86) are 

 employed. When the object has been thus brought fully into view, the 



