ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 419 



located. It is well to have an intermediate stamping ground for tem- 

 porarily depositing the selected diatoms where they can be cleared from 

 adhering fragments before finally locating them. It is important to 

 have all these cover-glasses of the same thickness, so that no time will 

 be lost in focusing as we pass from one to the other. 



One difficulty in picking up from a diatom spread will be found due 

 to adhesion to the glass surface. This adhesion is caused by dissolved 

 silica, or by some other soluble substance which may come from the 

 glass surfaces, or from the water in which the diatoms are suspended. 

 This adhesion may be prevented in part, by repeated settlings from freshly 

 distilled water ; but at the best, some of the more delicate shells will 

 often break before they can be detached. 



The author has recently tried a plan which promises to be a help in 

 such cases. This plan is to grind down thin slips of baked pipeclay 

 or similar material. This material is then finely surfaced and ignited 

 to drive off moisture. These slips may be blackened by charring sugar 

 solution, which can be soaked into them. They are then scrubbed in 

 order to remove adhering particles, and again heated to drive off 

 moisture. The diatom-spreading pipette is then drawn across the 

 surface, and the liquid is at once absorbed before it has time to dry 

 on the surface and cause adhesion of the diatoms. Porous arc-light 

 carbon may be ground down thin and used for this purpose. Keflected 

 light must be used over these opaque surfaces. A two-thirds objective 

 will give sufficient working space, and the eye.-piece should be an erect- 

 ing one. 



Monobromide of naphthalin has usually been regarded as a difficult 

 mountant to make secure. Several of the monobromide mounts made 

 by the author, however, in 1895 have kept in good order up to this 

 date. For this reason it may be of interest to give some account of 

 the method used in preparing them. They were sealed with gelatin. 

 The refractive power of the unmixed monobromide used appears not 

 to have suffered by lapse of time. Loss of refractive power has been 

 reported to have occurred when a wax seal was employed. The glass 

 covers that have been coated with thin gelatin in the way that has 

 been described are next given a marginal ring, and the reception 

 slide itself is also given a preparatory ring. This treatment secures 

 reliable contact of gelatin and glass, as both gelatin rings are dried 

 before the monobromide is applied. A binding ring of warm gelatin 

 completes the adhesion of the two gelatin rings after the monobromide 

 has been included. After this sealing ring has dried and has proved to 

 be secure, and not till then, a final ring of shellac is applied. Any other 

 cement that will resist the action of immersion fluids can be used in place 

 of shellac. A recent sample of bleached lac has developed fine cracks 

 on drying. The older slides did not show these cracks in the rings. 

 The use of shellac is to protect the gelatin from damp. 



The gelatin preparatory rings were composed of gelatin 20 gr., 

 sugar 10 gr.,and water 2£ drams. These preliminary rings need not be 

 thicker than the height the diatoms make necessary. Experience has 

 shown that these rings should be broad, and that the surfaces of contact 



