446 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS 



prevent the varnish applied round its border from running in. 

 Where a somewhat deeper cell is required, Prof. H. L. Smith 

 (U.S.A.) suggests the following specially for the mounting of 

 diatoms. A sheet of thin writing-paper dipped into thick shellac 

 varnish is hung up to dry ; and rings are then cut out from it by 

 punches of two different sizes. One of these rings being laid on 

 a glass slide, and the cover, with the object dried upon it, laid on the 

 ring, it is to be held in its place by the forceps or spring-clip, and 

 the slide gently warmed so as to cause a slight adhesion of the 

 cover to the ring, and of the ring to the slide ; and this adhesion may 

 then be rendered complete by laying another glass slide on the cover 

 and pressing the two slides together, with the aid of a continued 

 gentle heat. Still deeper cells may be made with rings punched out 

 of tinfoil of various thicknesses and cemented with shellac varnish 

 on either side. And if yet deeper cells are needed, they may be 

 made of turned rings of vulcanite or ebonite, cemented in the same 

 manner. There is, however, a tendency in shellac-formed cells to 

 throw off a cloudiness inside the cell, usually called 'sweating/ 

 which is very undesirable. It has been found that a ring of solid 

 paraffin, to which the cover is attached, if first ' ringed ' with the 

 same material and afterwards with a finishing varnish, makes a 

 useful and permanently clean dry shallow cell ; or paper may be 

 saturated with paraffin and treated as described for shellac. 



Cement-cells. Cells for mounting thin objects in any watery 

 medium may be readily made with asphalte or Brunswick black 

 varnish by the use of Mr. Shadbolt's ' turn -table ' or one of its modi- 

 fications. The glass slide being placed under its spring in such 

 a manner that its two edges shall be equidistant from the centre (a 

 guide to which position is afforded by the circles traced on the brass), 

 and its four corners equally projecting beyond the circular margin 

 of the plate, a camel's-hair pencil dipped in the varnish is held in the 

 right hand, so that its point comes into contact with the glass over 

 whichever of the circles may be selected as the guide to the size 

 of the ring. The turn-table being made to rotate by the application 

 of the left forefinger to the milled head beneath, a ring of varnish 

 of a suitable breadth is made upon the glass ; and if this be set aside 

 in a horizontal position, it will be found, when hard, to present a very 

 level surface. If a greater thickness be desired than a single appli- 

 cation will conveniently make, a second layer may be afterwards 

 laid on. It will be found convenient to make a considerable number 

 of such cells at once, and to keep a stock of them ready prepared for 

 use. If the surface of any ring should not be sufficiently level for a 

 covering glass to lie flat upon it, a slight rubbing upon a piece of 

 fine emery paper laid upon a flat table (the ring being held down- 

 wards) will make it so. 



Ring-cells. For mounting objects of greater thickness it is 

 desirable to use cells made by cementing rings, either of glass or metal, 

 to the glass slides, with marine glue. Glass rings of any size, dia- 

 meter, thickness, and breadth are made by cutting transverse sections 

 of thick-walled tubes, the surfaces of these sections being ground 

 flat and parallel. Not only may round cells (fig. 375, A, B) of vari- 



