SLIDERS—MOUNTING. 
object, by which it will be seen as an object is on painted 
glass. 
Since, however, there are some few objects which cannot be 
rendered translucent, expedients must be provided, by which 
they can be illuminated upon that side of them which is presented 
to the microscope. It is often necessary, also, even in the case of 
translucent objects, that they should be viewed by means of light 
thrown upon that side of them which is turned to the object- 
glass. 
26. These general observations being premised, we shall pro¬ 
ceed to explain the method by which the optical part of the 
instrument is mounted, and the several accessories by which the 
object is supported, moved, and illuminated. 
Let us suppose, for the present, that the eye-piece e f, fig. 13, 
and the object-piece o, are mounted in a vertical tube, with 
whose axis AAA, the several axes of the lenses, accurately co¬ 
incide. Let d d be a diaphragm, or blackened circular plate, with a 
hole in its centre, placed in the focus of the eye-glass, by which 
all rays of light not necessary to form the image shall be inter¬ 
cepted. Let D be a milled head, by turning which the tube 
which carries the eye-piece can be moved within certain limits to 
and from the object-piece, and let d' be another milled head, by 
which the tube which carries the object-piece can be moved 
within certain limits to and from the object, or by which the 
entire body b b of the microscope, carrying the object-piece and 
eye-piece, can be moved to and from the object. 
27. Let s s be a flat stage of blackened metal or wood, having 
a circular hole in its centre, as shown in plan at s' s', and let it 
be fixed by proper arrangements, so that the axis A A A of the 
microscope shall pass through the centre of the circular aperture, 
and so that its plane shall be at right angle to that axis. Let a 
slider, such as we have described above, upon which an object is 
mounted, be laid upon this stage, so that the object shall be in 
the centre of the hole, and therefore in the axis A A A of the 
microscope. 
28. Let i M be a concave reflector, receiving light either from 
a lamp or a window, and reflecting it upwards towards the open¬ 
ing in the slider, in converging rays, so as to condense the light 
with more or less intensity upon the under side of the object; if 
the convergence produced by m m be insufficient, it may be 
augmented by the interposition of a convex lens c c. This may 
or may not be interposed, according as the object is smaller or 
greater, and requires a more or less intense illumination. 
The light being thus thrown upon the lower side of the object, 
the latter, being sufficiently translucent, is rendered visible by it. 
29 
