THE MICROSCOPE. 
at pleasure the object-piece from over the stage, to change or clean 
the lenses. The arm, 9, can be fixed in its position by the pin, 
whose milled head is 10. 
The instrument is focussed first by moving the body to and from 
the stage by means of the pinion, 7, and rack, 5, the adjustment 
being completed by a much slower motion imparted to the body 
by the milled head, 11, which is connected with a screw and lever, 
by one revolution of which the body is moved through the 
300th part of an inch. An elastic play is allowed to the body, so 
as to guard against injury by the accidental contact of the object- 
piece with the slide. 
The usual rectangular motions are imparted to the stage, 12, 
through the extent of an inch, by the milled heads, 13, which 
act on pinions, by which the racks are driven which carry the 
stage right and left, and backward and forward. The illumi¬ 
nating mirror, 14, is supported in the usual way, so as to be placed 
at any desired angle with the axis of the instrument. Below the 
stage is fixed an arm, 15, capable of being moved up and down 
by rack and pinion. This arm supports a tube, 16, intended to 
receive apparatus to modify the light transmitted by 14 to the 
object. Various apparatus for condensing and otherwise modify¬ 
ing the illumination are provided, which fit into this tube, 16. 
A motion of revolution round its axis is given to this tube by the 
milled head, 17. By these means, the effect of oblique light can 
be shown on all parts of the object. A condenser, 18, invented 
by Mr. Gillet, of a peculiar construction, provided with a series 
of diaphragms formed in a conical ring, is inserted beneath the 
stage. 
Polarising apparatus, and other appendages, can also be attached 
to the secondary stage. 
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