196 



THE MICROSCOPE. 



receive a vertical plate, to which a downward motion is 

 given by a single fine screw, surrounded by a spiral spring,, 

 which elevates the plate as soon as the screw-pressure is 

 removed. The vertical plate carries an arm at right angles 

 to its own plane, terminating in a square frame Z>, capable 

 of receiving very thin, or somewhat thicker glass. The 

 arm has likewise a horizontal motion, so that the upper 



Fig. 122. Boss's Compressorium. 



plate D can be turned completely off the lower one B. 

 Should the thin upper glass be broken, it can be instantly 

 replaced, as no cement is required ; it is merely needful to 

 remove the fragments and slip a fresh glass in. It often 

 happens that on account of the trouble attendant upon 

 the use of all ordinary Compressorium s, the microscopist 

 simply uses a slide and a piece of covering-glass ; but if 

 he wishes an exact means of regulating the pressure, 

 some such Compressorium as "Ross's should always be 

 employed. 



