ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 541 



Whatever may be the material of which the cell is made, whether 

 cement, or curtain ring, or hard rubber, file a little notch on the top 

 of it, and on what will be the lower side of the slide, with a thin 

 dentist's file, and large enough to lay a pin in the opening. Then, 

 when the object is secured in the cell, moisten the rim of it, except 

 in the notch, with (^anada balsam, and apply the thin glass cover. 

 After a day or two, balsam of the thickness of common syrup may be 

 applied around the edge in such quantity, more or less, as one may 

 fancy, care being taken to avoid the minute aperture. Or, if the 

 balsam should happen by accident to run into the opening, it may be 

 easily drawn out with a pin or needle applied a few times. With the 

 point of a knife on the turntable, first push off any balsam that may 

 be on the cover-glass, then gradually work it up on the slide until 

 the edge of the mounting is a true slope or curve as may be desired. 



Mr. Merriman uses for his own opaque mountings, cells made of 

 bleached shellac cement and thoroughly dried. They are translucent 

 and therefore well adapted for the parabolic reflectors. The balsam 

 in this case forms a beautiful finish, and there is no difficulty in 

 applying it without interfering with the aperture. He does not 

 regard wax cells as safe to use with any mountings, without first 

 being covered with some cement, such as liquid marine glue, or gold 

 size and dammar. If used for opaque mountings the notch should be 

 cut in them before being covered with cement. 



This device of leaving a minute aperture in the cells of opaque 

 mountings has been tested for the past three years, and there has not 

 been a failure in a single case that was owing to the mounting, while 

 in hermetically closed cells there would be haziness, or the running 

 of cement, or some trouble after a time with nearly half the speci- 

 mens. 



Copal Varnish.* — Mr. Deby finds this varnish dries very rapidly 

 if slightly heated, or even if placed on a previously warmed slide. 

 Dr. Van Heurck, of Antwerp, was the first to use it. The varnish to be 

 used is what is called the " pale copal," and its consistency ought to 

 be that of oil. It is much pleasanter to use than Canada balsam, 

 does not make bubbles, and its refractive index is not very different 

 from that of balsam, and does not interfere with the resolution of 

 diatom markings. He has lately made many preparations in copal, 

 dispensing with the cover-glass altogether. The drop of copal is 

 placed on the diatoms and heated lightly over the spirit lamp. It 

 soon takes the consistency of amber, and is hard enough to sustain 

 wiping and brushing with a soft brush with impunity. The optical 

 aberrations produced by the cover-glass are thus done away with. 



Test for Illuinination.| — Dr. Carl Seller recommends blood- 

 corpuscles as the best test for proper illumination. He says : — 



" In arranging our illuminating apparatus, we must have some- 

 thing to judge of the quality of the light as it passes through the 

 Microscope ; and it will be found that there is no better test than 



* Journ. Quek. Micr. Club, vi. (1880) p. 167. 

 t ' Compendium of Microscopical Technology,' i.p 1+, 15. 17. 

 Ser. 2.— Vol. I, " 2 



