MOUNTING OBJECTS TN FLUID. CEMENT-CELLS. 255 



There are some objects, however, whose parts are liable to be 

 displaced by the slightest shifting of this kind ; and it is more 

 easy to avoid making air-bubbles by watching the extension of the 

 fluid as the cover is lowered, and by introducing an additional 

 supply when and where it may be needed, than it is to get rid of 

 them afterwards without injury to the object. When this end has 

 been satisfactorily accomplished, all that is needed is, first to 

 remove all superfluous fluid from the surface of the slide, and from 

 around the edge of the cover, with a piece of blotting-paper, 

 taking care not to draw away any of the fluid from beneath the 

 cover, or (if any have been removed accidentally) to replace what 

 may be deficient ; and then to make a circle of asphalte or gold- 

 size around the cover, taking care that it 'wets' its edges, and 

 advances a little way upon its upper surface. When this first coat 

 is dry, another should be applied, particular care being taken that 

 the cement shall fill the angular furrow at the margin of the cover. 

 In laying on the second coat, it will be convenient, if the cover be 

 round, to make use of the Turn-table (Fig. 103) ; and if the slide 

 be so carefully laid upon it that the glass-cover is exactly con- 

 centric with its axis, the turn-table may be used even for the first 

 application of the varnish, though a slight error in this respect 

 may occasion the displacement of the cover. — By far the greater 

 number of preparations which are to be preserved in liquid, how- 

 ever, should be mounted in a Cell of some kind, which forms a 

 well of suitable depth, wherein the preservative liquid may be 

 retained. This is absolutely necessary in the case of all objects 

 whose thickness is such as to prevent the glass-cover from coming 

 into close approximation with the slide ; and it is desirable when- 

 ever that approximation is not such as to cause the cover to be 

 drawn to the glass-slide by capillary attraction, or whenever the 

 cover is sensibly kept apart from the slide by the thickness of any 

 portion of the object. Hence it is ouly in the case of objects of 

 the most extreme tenuity, that the Cell can be advantageously dis- 

 pensed with ; the danger of not employing it, in many cases in 

 which there is no difficulty in mounting the object without it, 

 being that after a time the cement is apt to run-in beneath the 

 cover, which process is pretty sure to continue when it may have 

 once commenced. 



166. Cement-Cells. — "When the cells are required for mounting 

 very thin objects, they may be advantageously made of varnish 

 only, by the use of the Turn-table (Fig. 103) contrived by Mr. 

 Shadbolt. This consists of a small slab of mahogany, into one 

 end of which is fixed a pivot, whereon a circular plate of brass, 

 about three inches in diameter, is made to rotate easily, a rapid 

 motion being given to it by the application of the forefinger to 

 the milled-head seen beneath. The Glass slide being laid upon 

 the Turn-table, in such a manner that its two edges shall be equi- 

 distant from the centre (a guide to which is afforded by a circle 



Q 



