348 ACCESSORY APPARATUS 



when replaced by revolving the ring at E. any desired amount of 

 compression may be obtained. The object having been arranged, 

 either side may be examined with equal facility, as the compressor 

 is reversible. 



When a very small object is to be examined a small circular 

 cover-glass should be cemented with Canada balsam to the lower 

 cover-glass, and the object is thus confined to the centre of the field. 

 The zoophyte trough is a larger live-box differently constructed. 

 The form that has proved one of the best up to our own day was 

 introduced by Mr. Lister in 1834, and is well known. It is depicted 

 in fig. 302, being formed of slips of glass, and has a loose horizontal 

 plate of glass equal to the inside length of the trough, so that it 

 may be moved freely within it, also a slip of glass that will lie on 

 the bottom and fill it, with the exception of the thickness of this 

 loose plate. To use it, the slip is put upon the bottom, the loose 

 plate is placed in front of it with its bottom edge touching the 

 inside of the front glass, a small ivory wedge is inserted between 

 the front glass of the trough and the upper part of the loose vertical 



plate, which it serves to press 

 backwards ; but this pressure 

 is kept in check by a small 

 strip of bent whalebone, 1 

 which is placed between the 

 vertical plate and the back 

 glass of the trough. By 

 moving the ivory wedge up 

 and 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 



FlG 302 their lower margins are always 



in close apposition, it is evi- 

 dent the one will incline to the other with a constant diminution 

 of the distance between them from above downwards. An object 

 dropped into this space will descend until it rests between the two 

 surfaces of glass, and it can be placed in a position of great conveni- 

 ence for observation. 



By very little contrivance these troughs with their contents may 

 be kept, when not under examination, in much larger aquaria, ob- 

 taining the advantage of aeration and coolness. 



Mr. Botterill devised a trough which is made of two plates of 

 vulcanite or metal which screw together, and between them are two 

 plates of glass, of the proper size, of any desired thickness, kept 

 apart by half a ring of vulcanised indiarubber, the whole being 

 screwed tightly enough together by three milled heads to prevent 

 leakage. But leakage or the fracture of glasses is not uncommon 

 with this otherwise convenient form. 



An excellent, though shallow, trough was made by Mr. C. G. 

 Dunning, which we illustrate in fig. 303. The lower plate or trough 



1 Watch-spring or other elastic metal should not be used, on account of oxidation. 



