136 SELECTION AND USE 



over the liquid, for the purpose of forming a layer of equal 

 depth throughout, and if there should be any danger of crush- 

 ing the objects, a few fibres of hair, silk, cotton, etc., will keep 

 the two thin glasses sufficiently apart. The edge of the cell 

 having been lightly smeared with a soft mixture of beeswax and 



Fig. 35. CELIrTROUGH. 



oil. and the interior corner having been moistened with pure 

 water by means of a camel-hair pencil, the inverted cell is 

 placed on the large cover, which at once adheres, and thus the 

 cell is converted into a veritable Wardian case. Owing to the 

 moist atmosphere, there is no danger of the liquid under exam- 

 ination drying up, and as the apparatus is very cheap, several 

 may be fitted up and used for watching the life history of any 

 particular subject. 



The Compressorium. The animalcule cage, shown in 

 Figure 30 is open to the objection that it is difficult to 

 adjust the pressure with sufficient delicacy, and we are apt 

 either to crush the animal or leave it too free in its movements. 

 These difficulties may be avoided by the use of a well-made 

 Compressorium, of which there are several kinds in use. 



This little instrument also serves another important purpose. 

 In the examination of certain objects, it is frequently necessary 

 to flatten, and even to crush them, in order to render their 

 structure visible, and this the compressorium enables us to ac- 

 complish. The most powerful instrument of this kind consists 

 of a metal plate, in the centre of which is fixed the disc of glass 

 upon which the object is laid. A second disc of glass, fastened 

 in a ring which is hung at the end of a lever, by means of two 

 pivots, is pressed against the first by means of a screw, which 

 tilts the lever. In this way a very strong pressure may be 

 exerted, while, o'vying to the free movement of the ring on the 

 pivots, the plates of glass always remain parallel to each other. 



Where very great pressure is not required, a different form 

 of the instrument may be used. Instead of being forced down 

 Tbv a scrw uncl lever, the upper glasg clisc is fastened tq a thin 



