138 PEACTICAL TREATISE ON THE MICEOSCOPE. 



circular nut ; and two wedge-shaped tongues of watch spring 

 are placed between the spiral springs and this plate. These 

 tongue-shaped springs are capable of being moved round upon 

 the rods, and are for the purpose of communicating pressure 

 to a thin plate of glass resting upon the plate, which is pre- 

 vented from sliding off by a raised edge. The plate carrying 

 the thin glass cover is capable of being raised or depressed at 

 wiU, by means of the circular nut. It wiU be seen, that 

 when the plate carrying the thin glass cover is raised up as 

 high as it will go by the mUled nut, the cover will not touch 

 the lower plate of glass ; when this is the case, the instrument 

 is ready for the reception of an object. The ends of the little 

 steel springs must be lifted up by the finger-naU or some 

 thin instrument, and then rotated so far outwards, as to 

 get them clear of the cover. The cover being lifted off, the 

 object is to be placed upon the bottom plate with as much 

 fluid as necessary, and the cover being replaced, the springs 

 may be lifted up and turned back to their original position. 

 If now the nut be screwed down, the spiral springs will cause 

 the plate to follow the nut, and when the nut has been turned 

 far enough to allow the cover to come In contact either with 

 the object or the fluid, it wiU be noticed that as the screwing 

 is being proceeded with, both the fluid and the object wiU 

 be more and more flattened, until it arrives at a maximum. 

 If the screwing be continued further, the nut will leave the 

 plate carrying the thin glass cover, and the cover itself 

 wiU remain pressed down upon the object-plate with all the 

 force exerted by the spiral and by the tongue-shaped 

 springs. 



Mr. E.OSS has improved upon the compressorium of Mr. 

 Lister, by making the plate carrying the thin glass cover, 

 square, and by adding to it two other pillars, making four in 

 all; upon two of these, situated at opposite corners, strong 

 spiral steel springs are wound, and to the two others are 

 applied finger-shaped pieces of German silver, to keep down 

 the thin glass cover. The action of the large nut is the same 

 as in Mr. Lister's instrument, but the pressure exerted by the 

 springs is more powerful than in it. The finger-shaped pieces 



