150 ACCESSORY APPARATUS. 



inside bottom of the trough, but whose breadth is inferior by the 



thickness of the plate just men- 

 tioned. The trough being filled 

 with water (fresh or salt, as the 

 case may be), the horizontal slip is 

 laid at the bottom, and the vertical 

 plate is placed in contact with the 

 front of the trough, its lower mar- 

 gin being received into the space 

 left at the front edge of the hori- 

 zontal slip, which serves to hold it 

 there, acting as a kind of hinge ; a 

 zoophyte Trough. small ivory wedge is then inserted 



between the front glass of the trough 



and the upper part of the vertical plate, which it serves to press 

 backwards ; but this pressure is kept in check by a little spring of 

 bent whalebone, which is placed between the vertical plate and 

 the back glass of the trough. By moving the ivory wedge up or 

 down, the amount of space left between the upper part of the 

 vertical plate and the front glass of the trough can be precisely 

 regulated ; and as their lower margins are always in close appo- 

 sition, it is evident that the one will incline to the other, with a 

 constant diminution of the distance between them, from above 

 downwards. Hence a Zoophyte, or any similar body, dropped 

 into this space, will descend until it rests against the two sur- 

 faces of glass, and will remain there hi a situation extremely 

 convenient for observation ; and the regulating-wedge, by in- 

 creasing or diminishing the space, serves to determine the level 

 to which the object shall fall. Of these troughs, again, it is con- 

 venient for the working Microscopist to be furnished with 

 several, of different sizes ; and in one of them OJiara or Nitella 

 may be kept growing in a state very convenient for observation. 

 A similar trough may be provided for this last purpose, however, 

 by dispensing with the vertical plate and horizontal slip alto- 

 gether, and approximating the front and back plates so that only 

 a very narrow space is contained between them ; in this case it 

 is convenient to let the upper lip of the back plate project con- 

 siderably beyond that of the front plate, as objects may then be 

 much more readily inserted between them ; and the back plate 

 may also be conveniently made to project beyond the sides of 

 the trough, as would be useful, too, in the case of larger troughs. 

 If it be wished to grow Chara, &c., in a thin trough of this kind, 

 the trough, whenever it is not under observation, should be im- 

 mersed in a tumbler or jar of water, since the plant will not 

 nourish in a very limited supply. 



70. Compressor ium. The purpose of this instrument is to 

 apply a graduated 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 



