154 SUMMARY OF CURRENT RESEARCHES RELATING TO 



position, the slide may be heated a little, up to 45°, and when rearranged, 

 allowed to cool. Glycerin is then added to the preparation, the cover- 

 glass imposed, and the preparation fixed up in the usual way. 



Limpid Copal Solution.* — A limpid and colourless solution of gum 

 copal has long been a desideratum to microscopists, and Dr. F. L. 

 James has spent many hours in trying to obtain one. The follow- 

 ing process he found originally in a German journal, ' Der Techniker,' 

 and having given it a fair trial, can say that if a high grade of bright 

 copal is chosen, the product will be perfectly limpid and almost colour- 

 less. By sorting the copal, a solution as limpid as water may be obtained. 



Dissolve 4 parts of camphor in 48 parts of sulphuric ether and add 

 16 parts of pulverized gum copal thereto. Cork the flask carefully, and 

 Btand aside with occasional agitations until the copal is partly dissolved 

 and partly swollen to its fullest extent. Then add 16 parts of alcohol 

 of 96° and 1 part rectified oil of turpentine, and agitate thoroughly. 

 Let stand with occasional agitations for several days, and at the expira- 

 tion of a week or so, the contents of the flask will be found to have 

 separated into two layers, of which the lower is rather dark, thick, and 

 possibly dirty, according to the quality of the copal, but above this a 

 layer will be found rich in copal and as clear as crystal itself. The 

 lower layer may be further treated with camphor and sulphuric ether, 

 and afterwards with alcohol, and made to give a still further yield of 

 the crystalline fluid. The only objection to this solution of copal is 

 that it is somewhat brittle when dry. This may be obviated by adding 

 a few drops of purified nut or poppy oil thereto. 



Preserving-fluids for Fleshy and Succulent Plants, f — Herr E. 

 Sadebeck recommends for this purpose a 4-5 per cent., i. e. a nearly 

 saturated solution of barium-lead-nitrate, the object retaining its colour 

 in it for one or two months, while the solution itself remains clear. 

 Another good preserving-fluid for similar objects is a solution of cor- 

 rosive sublimate of a • 1 per cent, concentration, to which a few drops of 

 hydrochloric acid have been added. Boracic acid does not prevent decay, 

 even in a saturated solution. For Fungi which contain but little soluble 

 matter, a 20 per cent, solution of alcohol may be recommended. 



Determining the Thickness of Cover-glasses of Mounted Prepara- 

 tions.J — Dr. S. Czapski gives the following method for ascertaining the 

 thickness of cover-glasses where the preparation is already mounted. 

 This is very desirable for high powers. The procedure presupposes the 

 possession of some cover-glasses, the thickness of which is known, and 

 that the head of the fine-adjustment screw is divided by radial lines. 



The upper and under surface are focused with an objective of • 6 to 

 • 9 aperture and central illumination, and the amount of turn given 

 to the fine-adjustment screw noted for each cover-glass ; of course it is 

 unimportant whether the exact value of the screw turn is known or not. 

 If the surfaces of the cover-glass do not present any obvious marks to 

 focus on, an artificial one, such as dust or scratches, must be supplied. 

 If the numbers thus obtained be compared with the known real thickness 

 of the covers, a reduction factor is obtained from their quotients, which 

 is available for determining measurements of a similar kind, that is to say 

 for measurements of other cover-glasses with the same objective, ocular, 



• St. Louis Med. and Surg. Journ.. Iv. (1888) p. 231. 



t SB. Gesell. Bot. Hamburg, iii. (1887) p. 61. See Bot. Centralbl., xxxvi. (1888) 

 p 128. I Zeitschr. f. Wiss. Mikr , v. (1888) pp. 482-4. 



