140 PRACTICAL TREATISE ON THE MICROSCOPE. 



much stouter, and fits accurately upon the raised object- 

 plate. 



Troughs for Chara and Polyps. — These consist of two 

 plates of glass cemented together, with strips of the same 

 material, or of metal between them, to form the sides of the 

 trough; one of these, as described by Mr. Varley, in the 

 forty-eighth vol. of the Transactions of the Society of Arts, is 

 represented by fig. 93. c is a bottom-plate of stout glass, upon 



which is cemented with pitch 

 and bees'-wax a thin cover, d, 

 with slips of glass between it 

 and the bottom-plate, to form 

 the sides. The cover, d, is not 

 so broad as the plate, c, in order 

 Fig. 93. that a slip of chara may be 



more readily placed in the 

 trotigh, as it can be first laid upon c, and then gradually 

 slid down between it and the cover, d. In order to render 

 the trough more manageable, it may be cemented to a larger 

 bottom-plate, a b, by Canada balsam ; but it will be found far 

 more advantageous if the bottom-plate itself be as large and 

 as broad as a h, and if the cover, d, be cemented to it and not 

 to another plate, as then two extra surfaces will be dispensed 

 with. Mr. Varley informs us that a piece of wire bent into 

 the shape of the slips of glass represented in the figure, and 

 covered thickly with a cement composed of bees'-wax and 

 pitch, will form an excellent substitute for the slips, and look 

 very neat ; the cements of Canada balsam or sealing wax are 

 much too brittle to last long, as a sudden jar will cause them 

 to give way. Messrs. Smith and Beck supply with their 

 microscopes a larger and much thicker trough for chara and 

 polyps, as represented by fig. 94 ; the front is composed of 

 much thinner glass than the back, and the method adopted of 

 confining objects near to the front varies according to circum- 

 stances. One of the most convenient plans, is to place in the 

 trough a piece of glass that will stand across it diagonally, as 

 represented by fig. 94, and if the object be heavier than 

 water, it will sink, until it is stopped by the diagonal plate. 



