J. DEBY "receipts FOR MICROSCOTISTS." 107 



ordinarj glass slip with a circular hole, say half an inch or more in 

 diameter in the middle ; lay this slip on an ordinary glass slide, not 

 perforated. Then grease the top of tlie upper or perforated slide 

 just a little way around the circular hole, and join the two slips of 

 glass by means of two rubber rings. The object is then placed on 

 a thin cover-glass, somewhat larger than the hole in the slide ; it is 

 then covered by a thin glass cover, ^ inch in diameter ; the whole 

 is then turned down and fastened to the slide by the adherence with 

 the grease, while the small cover prevents the running of the liquid. 

 The plant or animal under examination finds itself confined in a sort 

 of miniature Ward's case. When not under observation, the grow- 

 ing slide is laid flat in a shallow plate with water just above the line 

 of junction of the two slips of glass, where, by capillarity, it creeps 

 up to the central cell, where evaporation keeps the contained 

 atmosphere in a state of constant and healthy saturation. (See 

 Plate XIT, lower portion.) 



VI. — Copal Varnish. I find this varnish dries very rapidly 

 if slightly heated, or even if j)laced on a previously warmed slide. 

 I have many hundred slides of diatoms prepared in copal varnish, 

 and my friend, Mr. Van Heurck, of Antwerp, who was the first to 

 use this material, has many thousands. 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 solution of diatom 

 markings. I have of late made many preparations in copal dis- 

 pensing with the cover glass altogether. The drop of copal is placed 

 on the diatoms and heated lightly over the spirit lamj^. 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. 



I hope at some future day to add a few more '' Receipts for Micro- 

 scopists " to the above list ; may the above, however, in the mean- 

 time lead to similar communications from brother microscopists. 



* This varnish can be obtained from Wallis & Co., 64, Long Acre, London. 

 It is very much cheaper than balsam. 



JouRN. Q. M. C.J No. 45. 



