OF THE MICKOSCOPE. 131 



Watch-Glasses. Dr. Beale recommends small flat watch- 

 glasses for holding liquids that are to be examined, and we 

 Lave found them very excellent. The best kind for this pur- 

 pose are those known as lunette glasses, which are nearly flat on 

 the bottom. They are awkward things to manipulate, how- 

 ever, unless some means is provided for holding them steady, 

 and moving them about on the stage. We use for this purpose 

 a strip of wood, three inches long, and so wide that we can 

 easily bore in it a hole, about one-eighth of an inch less in 

 diameter than the watch-glass, of which the smallest size should 

 be chosen. The thickness of the strip should be sach that 

 when laid on any flat surface, the watch-glass will not come in 

 contact with it. Glasses held in this way are very convenient. 



Fig. 29. WATCH-GLASS HOLDER. 



Watch-glasses are very convenient for examining a "dip " from 

 a pond or stream, but for this purpose they require a holder. 

 The little instrument shown in the cut is formed of three pieces 

 of bright tin, which are hinged together. In the upper piece 

 is cut a hole just large enough to receive a watch-glass. A ring 



