PLATE GLASS CELLS. 237 



main entire after the cutting of disks (§ 117) may be employed, 

 the disks often falling out of themselves when the glass is laid 

 aside for a few days ; and thus the same piece of thin glass may 

 aiford a plate, which, when cemented to a glass slide forms a 

 cell, and a disk suitable as the cover to a cell of somewhat 

 smaller size. There is great danger, however, of the cracking of 

 the surrounding glass, especially when the disk is of large size ; 

 and it will generally be found a saving of trouble, to employ the 

 method recommended by Dr. L. Beale. This consists in attach- 

 ing a piece of thin glass to one of the glass rings of which the 

 deeper cells are made (§ 137), of any form that may be desired, 

 by means of marine glue, first laid upon the latter, and melted 

 upon the hot plate ; when the glue is quite cold, the point of a 

 round or semicircular file is sharply thrust through the centre of 

 the thin glass, which is carefully filed to the size of the interior 

 of the ring ; and the ring being then heated a second time on the 

 hot plate, the thin glass plate may be readily detached from it, 

 and at once cemented upon the glass slide. The success of this 

 simple process depends upon the very firm and intimate ad- 

 hesion of the thin glass to the ring, which prevents any crack 

 from running into the part of the thin glass that is attached to 

 it, however roughly the file may be used. By having many of 

 the rings on the hot plate at once, and operating with them in 

 turn, a great number of cells can be made in a short time ; and 

 such large thin cells may be made in this mode, as could scarcely 

 be fabricated (on account of the extreme brittleness of this glass) 

 by any other. A press, consisting of two plates of brass screwed 

 together, holding the thin glass between them, has been devised 

 by Mr. C. Brooke for the same purpose ; but the foregoing 

 method has the advantage, not only of requiring no special ap- 

 paratus, but also of enabling the form and size of the perforation 

 to be readily varied. After the thin glass has been cemented to 

 the slide, it is desirable to roughen its upper surface, by rubbing 

 it upon a leaden or pewter plate (§ 108) with fine emery ; since 

 the gold-size or other varnish adheres much more firmly to 

 a " ground" than to a polished surface. Although the thin 

 glass cell requires much more trouble in its preparation than the 

 cement cell, yet it is decidedly to be preferred for any very 

 choice objects; since, if any air should find admission, it is more 

 readily detected ; and the remounting of the object may be 

 accomplished in the same cell, with very little disturbance of its 

 position. 



186. Plate Grlass and Shallow Cells. — ^For mounting objects of 

 somewhat greater thickness than can be included within thin glass 

 cells, shallow cells may be made by drilling apertures of the de- 

 sired size in pieces of plate-glass of the requisite thickness, and 

 by attaching these with marine glue to glass slides (Fig. 64). 

 Such holes may be made not merely circular (a) but oval (c) ; 

 and a very elongated perforation may be made, by drilling two 



