CHARA AND POLYPE TROUGHS. 197 



Smith and Beck's trough for chara and polypes, a sec- 

 tional view of which is shown at fig. 123, is made of three 

 pieces of glass, the bottom being a thick strip, 

 and the front a of thinner glass than the back 

 b ; the whole is cemented together with Jeffery's 

 marine-glue. The method adopted for confining 

 objects near to the front glass varies according 

 to circumstances. One of the most convenient 

 plans is to place in the trough a piece of glass 

 that will stand across it diagonally, as at c; then 

 if the object be heavier than water, it will sink, 

 until stopped by this plate of glass. At other 

 times, when used to view chara, the diagonal 

 plate may be made to press it close to the front 

 by means of thin strips of glass, a wedge of glass 

 or cork, or even a folded spring. When using <^ 

 the trough, it is necessary that the microscope Fi s- 12S - 

 should be in a position nearly horizontal. 



Mr. Walker's trough for exhibiting the circulation of 

 the blood in a fish's tail, &c. (fig. 117 a) consists of a piece 

 of plate-glass about 6 inches long, and 2 inches wide ; 

 upon this, three other pieces of plate-glass, about \ 

 inch wide, are cemented with marine-glue. Three pieces 

 of strong covering glass, about a \ inch wide, are 

 also cemented on to the plate, and a piece of mode- 

 rately strong covering glass on to the top of these thin 

 slips ; a piece of plate-glass is then cemented to the top 

 of the thin glass, abutting on the ends of the slips ; the 

 whole forming an open trough, terminating in a thin cell, 

 which is closed at all parts, except where it communicates 

 with the bottom of the trough. 



The fish, wrapped in a little wet linen, is placed in the 

 trough, and the tail is thrust flat into the thin cell ; a 

 small quantity of water is placed in the large trough, and 

 the fish kept in its place by one or two elastic bands or 

 strips of thin sheet-lead passed round the glass and over 

 the body of the fish. 



The advantages of this arrangement are : — 1st. The 

 fish cannot throw up its tail and splash the object-glass 

 with water. 2d. In consequence of the tail being kept 

 flat in the cell, the view is perfect, and there is little risk 



