THE MICROSCOPE. 
The most simple, and consequently the cheapest description of 
mounting, is that in which fewest parts are moveable. The only- 
parts of a compound microscope which are necessarily moveable 
are those by which the instrument is focussed, and the object 
illuminated. The most simple mechanical expedient for effecting 
the former is a rack and pinion attached either to the body or the 
stage, and for the latter the suspension of the reflector upon an 
horizontal axis, so that it can be inclined at any desired angle to- 
the axis of the body and the stage. 
Whatever be the form or disposition of the stand, it is- 
essential that the axis of the object-piece should pass through 
the centre of the stage, and that the reflector should be so set as 
to be capable of reflecting light in the direction of this axis. The 
body is generally a straight tube, the axis of the eye-piece and 
object-piece being in the same straight line. In the case of 
instruments mounted after the model of Professor Amici, however,, 
the body consists of a tube having two parts with their axes at 
right angles, the axis of the object-piece being vertical, while 
that of the eye-piece is horizontal. In this case, a prism is fixed 
in the angle of the tube, at an angle of 45° with the axes by 
which the rays proceeding vertically from the object-piece are- 
reflected horizontally to the eye-piece, on the principle already 
explained (30). 
75. One of the most simple models for the mounting of a com¬ 
pound microscope was contrived by Frauenhoffer so early as- 
1816, long before achromatic lenses were produced. This model, 
owing to its great simplicity, convenience, and cheapness, is still 
extensively used for the lower priced instruments, especially by 
the continental makers. 
The body of the instrument is attached to a vertical pillar, fig. 
36, p. 49, and its axis is permanently vertical. It is focussed by 
a rack and pinion, worked by a milled head on the right of the 
observer. The stage is fixed in its position, and placed on the 
top of a short tube, in the lower part of which the reflector is. 
suspended on an horizontal axis, so that it can be placed at any 
desired obliquity to the axis of the instrument, and * thus can. 
always throw a beam of light upwards to the object. One side 
of this mirror is concave, and the other plane. 
For the illumination of opaque objects, a lens is attached by 
a jointed arm to the upper part of the pillar, on which the 
instrument is supported. 
M. Lerebours, of Paris, makes excellent microscopes on this 
model, with a triple achromatic object-piece and other accessories,, 
which he sells at the very moderate price of 90 francs (3/. 125.).. 
Several thousands of these have been sold. 
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