COMPRESSOR. 
by different makers, according to the demands of different 
observers. 
One of tbe most common and useful forms of compressor is 
shown in fig. 22. 
A small and very thin disc of glass is set in a brass ring i, and 
supported at two points l l, diametrically opposite, by the ends of 
a fork L p, attached to 
a lever p g, the latter 
being supported upon 
two upright pieces r, 
attached to an hori¬ 
zontal piece p d. This 
piece F d turns hori¬ 
zontally round a pivot, 
fixed near the end E 
of a strong slip of brass A B, having the form and magnitude of a 
slide used for the support of objects. At the middle c, of A b, is 
a circular hole, in which another disc of glass is set, correspond¬ 
ing in magnitude to the disc I. A screw, with a milled head k, 
works in the end g of the lever, by turning which in one way or 
the other, the end g, and consequently the disc I, is raised or 
depressed. 
To place the object for observation, by moving the piece D 
round the pivot the ring i is removed from the lower disc o, 
upon which the object is then deposited. The screw K being 
turned, so as to raise the disc I sufficiently to prevent it from 
touching the object, the piece d is then turned on the pivot 
until the disc i is brought over the object. The observer 
then viewing the object in the microscope, and placing 
his hand upon the screw K, slowly turns it, so as gradually to 
compress the object, and continues this process or suspends it, or 
turns the disc i horizontally, so as to roll the object between the 
glasses, according as his course of observation may require. 
The compression may be so increased as to flatten the object, 
which in some cases is desired, so as to render it more transparent, 
while nevertheless its form becomes more or less distorted. 
42. It is sometimes required to ascertain the effects of an 
electric spark or voltaic current, transmitted through a liquid or 
solid, or through a body animate or inanimate. An apparatus 
adapted for this purpose is shown in fig. 23, where D c is a disc 
of glass set in the middle of a slip of brass A B. The two brass 
tubes g g play upon the hinges F f, which are supported on short 
glass pillars e e. Two glass tubes, through the bores of which 
fine platinum wires a a pass, are inserted tightly into the tubes 
g g, so that they can be pushed to, or drawn from the disc d, 
41 
