48 GENERAL HISTORY OF 



to its box by a screw, or »ome other convenient means, 

 as that a small body placed under it may receive a certain 

 degree of pressure without its parts being dislocated. In 

 my original live-boxes, this was effected by a screw being 

 attached to the cover; but, in the ordinary way, the cover 

 is made to revolve. In some, a guide-piece has been sub- 

 stituted for the screw, so that the pressure is obtained 

 without the glass-plates sliding one upon the other. The 

 German opticians attach the cover or upper plate to a 

 jointed lever, at the longest end of which a screw is ap- 

 plied, which brings the upper plate connected with the 

 short-arm, in contact with the lower plate. The use of 

 the crush-box is to protrude certain parts of the animal- 

 cule for examination by pressing down upon the creature. 

 In this manner, the teeth of the Rotatoria become dis- 

 tinct. Other uses of this apparatus are given when speak- 

 ing of the minute loricated Polygastrica. 



Section XX. — On Viewing Infusoria by Polarised Light, 



Having in the last edition of the Microscopic Illustra- 

 tions given a full description of the Polarizing Microscope, 

 and the apparatus necessary for using any microscope for 

 polarising purposes, a very few remarks on the effects 

 produced by viewing Infusoria under this light will be 

 sufficient here. 



The siliceous covering of Infusoria is but slightly affected 

 by polarised light : that the effect is only feeble, is attri- 

 butable to the extreme tenuity of their shells, for could 

 we but contrive the means of magnifying the effect, I feel 



