98 MICROSCOPICAL TECHNIQUK 



bear in mind that the cedar-oil, used with immersion objectives, is 

 a powerful solvent, and may irretrievably damage valuable slides 

 finished with a varnish, containing ingredients soluble in the oil. 

 Since it is generally found advisable, for the sake of uniformity, to 

 use one and the same varnish always, this should be selected with 

 due regard to the possible use of cedar-oil as an immersion fluid. 

 One of the simplest varnishes to make and use — sealing-wax 

 varnish — is also unaffected by cedar-oil. It should be made by 

 dissolving the finest sealing wax (preferably black) in methylated 

 spirit, and of such a consistency as to flow easily from the brush. 

 Before applying the varnish, the glass cover should be firmly 

 cemented with caoutchouc cement (Beale's formula, dissolve 

 separately equal parts of bottle india-rubber and shellac in mineral 

 naphtha to desired consistence), or the Gram-Riitzou composition 

 (Canada balsam, 5 gm ; shellac, 5 gm. ; absolute alcohol, 5 gm. ; 

 ether, 10 gm.). When the cement is quite dry, and not before, 

 apply a thin coat of the varnish. A somewhat important point is 

 the kind of brush to use. Best of all are sable " writing pencils," 

 " crow," and " small duck " being suitable-sized quills. These do 

 not come to a point when in use, and form perfect rings with less 

 trouble than those that do. They should be carefully washed 

 after use in alcohol, mineral naphtha, or turpentine, according to 

 the nature of the cement or varnish employed. — Pharm. Journal. 



Micro-Photographic Drawings.— Unna, the eminent dermato- 

 logist of Hamburg, suggested in 1892 a method of making repro- 

 ductions of micro-organisms, which is much superior to the 

 ordinary methods of either drawing or photography, combining the 

 accuracy of the latter with the clearness and comprehensiveness of 

 the former. The method is as follows : — 



From properly stained specimens negatives are made. From 

 these negatives light prints are made on soft paper, upon which it 

 is possible either to draw or paint without further preparation. 

 The photographs thus obtained give only the outlines of the 

 object or a skeleton of the picture which it is intended to produce. 

 By the aid of the micrometer screw of the microscope, the 

 appearance presented in the various strata of the specimen may 

 be easily sketched in by an artist. A more complete picture may 



