INFtrsORIAL ANTMALCUtES, 107 



by a screw, or some other convenient means, as that a small body 

 placed \mder 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, I have substituted a guide- 

 piece 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 applied, which brings the upper plate connected with the 

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

 box is to protrude certain parts of the animalcule for examination by 

 pressing do^vn upon the creature. In tliis manner, the teeth of the 

 Rotatoria become distinct. Other uses of this apparatus are given 

 when speaking of the muiute loricated Pohjgastrka. 



I have lately employed with very great advantage, in place of the 

 Aquatic Uve-box, an apparatus which is peculiarly siiited for viewing 

 living Infusoria. It combines the use of a live-box and compressor, 

 and may not inappropriately be called a7i Aquatic plate. It is com- 

 posed of a stout plate of glass, the size of a common slide. Over 

 this, is a moveable plate of brass, with a spring fixed to it, and a thin 

 disc of glass in the centi'e, to cover the object. The two plates are 

 connected by two screws — one to hold them together, the other for 

 adjustment of distance. I am indebted to Mr. Wenham for the 

 original idea which I^have improved upon. 



Section VI. — On Vieiviyig Infusoria hj Polarised Light. — Having, 

 in the last edition of the Microscopic Illustrations, given a full 

 description of the Polarising 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 sihceous covering of Infusoria is but slightly affected by 

 polarised light : that the effect is ojily feeble, is attributable to the 

 extreme tenuity of their shells, for could we but contrive the means 

 of magnifjong the effect, I feel convinced that some very important 

 results would be obtained. The ribs or striae on the Navicula, assume 

 a slight tinge of colouring when the polariser and analyser are 

 parallel to each other ; but when they are crossed, owing to the few 



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