COMPRESSOR. 



by different makers, according to the demands of different 

 observers. 



One of the 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 F, 

 attached to an hori- 

 zontal piece F 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 c, 

 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 



