A SHALLOW TROUGH 



349 



proper is made of metal, 3 inches long by 1^ wide and about } \ } 

 thick, with an oval or oblong perforation in the centre, and the 

 under side is recessed, as shown in fig. 303, B. In this recess is fixed, 

 by means of Canada balsam or shellac, a piece of stout covering glass, 

 forming the bottom of the cell, the recess being sufficiently deep to 

 prevent the thin glass bottom from coming into actual contact with 

 the stage of the microscope or with the table when it is not in use. 

 Two pieces are provided near the bottom edge of the cell : the cover 

 (fig. 303, C) is formed of a piece of thin brass, rather shorter than 

 the trough, but about the same width ; it has an opening formed in 

 it to correspond with that in the trough, and under this opening is 

 cemented a piece of cover-glass. The cover-plate is notched at the 

 two bottom corners, ;md at the two top corners are formed a couple 

 of projecting ears. In order to use this apparatus it must be laid 

 flat upon the table, and filled quite full of water. The object to I it- 

 examined is then 

 placed in the cell, and 

 may be properly ar- 

 ranged therein ; the 

 cover is then lowered 

 gently down, the two 

 notches at the bottom 

 edges being first 

 placed against the 

 pins ; in this way the 

 superfluous water will 

 be driven out, and the 

 whole apparatus may 

 be wiped dry. The 

 capillary attraction, 

 assisted by the weight 

 of the cover, will be 

 found sufficient to 

 prevent any leakage ; 

 and the pins at the 



bottom prevent the cover from sliding down when the microscope is 

 inclined. This zoophyte trough possesses two important qualities : 

 first, it does not leak ; second, it is not readily broken without gross 

 carelessness. The shallowness may be overcome by placing an ebonite 

 plate with the required aperture between the two mounted glasses. 



Infusoria, minute alga?, &c., however, can lie well seen by 

 placing a drop of the water containing them on an ordinary slide, 

 and laying a thin piece of covering glass on the top ; and objects 

 of somewhat greater thickness can be examined by placing a loop 

 or ring of fine cotton thread upon an ordinary slide to keep the 

 covering glass at a small distance from it : and the object to be ex- 

 amined being placed on the slide with a drop of water, the covering 

 glass is gently pressed down till it touches the ring. Still thicker 

 objects may be viewed in the various forms of ' cells ' hereafter to 

 be described, and as, when the cells are filled with fluid, their glass 

 covers will adhere by capillary attraction, provided the superfluous 



FIG. 303. 



