WATEE-TROUGH. COMPEESSOEIUM. 133 



bottom and ends of the trough I I ; and a thin-glass cover 

 being cemented on them, the trough is complete. Small 

 troughs of this kind may be conveniently made from ordinary 

 Glass Slides cut into halves ; the three strips being cut from 

 one- half, and the other half, if thin enough, serving as the 

 cover. 



99. Compressorium. — The purpose of this instrument is to apply 

 a gradual pressure to objects whose structure can only be made out 

 when they are thinned by extension. For such as will bear tolerably 

 rough treatment, a well-constructed Aquatic Box may be made to 

 answer the purpose of a compressor ; but there is a very large 

 class whose organization is so delicate as to be confused or alto- 

 gether destroyed by the slighest excess of pressure ; and for the 

 examination of such, an instrument in which the degree of com- 

 pression can be regulated with precision is almost indispensable. 

 The Compressorium in most general use is that represented in 

 Fig. 80, of which the general plan was originally devised by Schiek 



Fig. 80. 



Compressorium. 



of Berlin, whilst its details have been modified by M. de Quatre- 

 fages, who has constantly employed this instrument in his elaborate 

 and most successful researches on the organization of the Marine 

 Worms. It consists of a plate of brass between 3 and 4 inches 

 long, and from 1£ to 1| inches broad, having a central aperture 

 of from | to f of an inch. This central aperture is covered on its 

 upper side by a disk of thin glass, which may be cemented to the 

 brass plate by Canada balsam ; and the under side of it is bevelled 

 away, so that the thickness of the edge shall not interfere with the 

 approach of the objective to its margin, when that side is made 

 the uppermost. Near one extremity of the plate is a strong vertical 

 pin, that gives support to a horizontal bar which turns on it as on 

 a swivel ; through the end of this bar that projects beyond the 

 plate, there passes a screw with a milled-head ; and at the other 

 end is jointed a second bar, against one end of which the screw 

 bears, whilst the other carries a frame holding a second disk of 

 thin glass. This frame is a small circular plate of brass, having 

 an aperture equal in size to that of the large plate ; to its under 

 side, which is flat, a disk of thin glass is cemented by Canada 



