and Laboratory Methods. 2501 



projected, the purpose of the experiment must be considered as well as the forn: 

 and size of the organism, especially when the specimen is an active animal or 

 one of the interesting motile plants. It may be desired to exhibit either the 

 general morphology and characteristic motions of the organism, which are, in 

 general, best seen under a relatively low power objective, or anatomical details 

 and functional activity of organs, which require a higher magnification. For 

 example, one may desire, at one time, to exhibit the movements, changes of 

 shape, somites, and setae of an earthworm ; and again, to demonstrate the ani- 

 mal's thin-walled crop, muscular gizzard with contained food and sand grains, 

 the pulsating dorsal vessel and hearts, and the nephridia. In the first case, a 

 comparatively large cell. Fig. 9, No. 11, and a large worm, or two, are used; 

 while, in the second case, a compressor. No. 15, or a life box. No. 10, and a 

 small transparent bodied worm under the influence of chloretone will give the 

 best results. The low power objective will have a much longer working distance 

 and may be used, therefore, with a deeper cell than is required by the short 

 working distance of the higher power lens. In medium and high power projec- 

 tion, therefore, shallow cells are absolutely necessary and are desirable, even 

 when using low powers, because in them moving animals are -more easily kept 

 in focus. 



Cells with sloping sides, e. g.. Fig. 9, Nos. 3, 7, and 13, are, in general, better 

 than those having straight sides as in Nos. 12, 17, 18, and 21, since the animals 

 can not find a hiding place close against the side of the cell, nor is there any 

 interference with the passage of light through the cell. Thin bodied animals, 

 e. g., planarian worms, leeches, and small larvae, should not be mounted in cells 

 with straight sides, which are better adapted to thick bodied, active species and 

 to non-motile species, e. g., spongilla, bryozoa, and macroscopic plants, as in the 

 evolution of oxygen. 



Very small organisms, ranging in size from bacteria to small water fleas, 

 need only the form of mount adapted to high power objectives. 



A. Directions for moimtiiig specimens of microscopic size, such as anuvbce, infu- 

 soria, bacteria, desmids, diatoms, yeast, oscillaria, spirogyra, vaucheria and similar 

 forms. 



These types are to be mounted in essentially the same manner as for ordi- 

 nary microscopical examination, only slight modifications being necessary to 

 adapt the method to the requirements of a projection microscope. Place the 

 organisms in a drop or two of clean hydrant water, or water in which they have 

 been living, on a clean polished plate glass slide, or, if this is not at hand, on a 

 slide selected for clearness and flatness. Polished plate glass slides are prefer- 

 able because their flatness and even thickness requires re-focusing less frequently 

 while the object is being moved about under high power objectives. Over the 

 objects place a three-fourths or seven-eighths inch thin, clean, selected cover- 

 glass, being careful to exclude air bubbles. Large cover-glasses hold more 

 water and prevent rapid destruction of the mount by drying. If there is just 

 enough water on the slide to fill the thin space between it and the cover-glass, 

 the mount may be turned up on its side without displacement of the cover ; but 

 if there is too much water, the cover will float off when the slide is tilted. To 



