THIN-GLASS CELLS. — SUNK CELLS. 227 



of thin-glass to one of the glass rings of which the deeper cells are 

 made (§ 170), 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 semi- 

 circular file is sharply thrust through the centre of the thin-glass, 

 which is then to be 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 adhesion 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 lings on the hot- 

 plate at once, and operating with them in turn, a great number of 

 ceils 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. After the 

 thin -glass has been cemented to the slide, itis desirable to roughen 

 its upper surface by rubbing it upon a readen or pewter plate 

 (§ 138) with fine emery ; since the gold-size or other varnish adheres 

 much more firmly to a ' ground ' than to a polished surface. In- 

 stead of thin-glass, thin rings of Tin may be employed (§ 171), 

 provided that the fluid used in mounting is not one that acts upon 

 that metal. 



168. Sunk and Plate-Glass Cells. — For mounting objects of 

 somewhat greater thickness than can be included within thin -glass 

 cells, shallow Cells may be made by grinding-out a concave (either 

 circular or oval) in the thickness of a glass plate (Fig. 104.) An 

 a priori objection naturally suggests itself to the use of such cells, 

 — that the concavity of their bottom will so deflect the course of the 

 illuminating rays as to distort or obscure the image ; but to this it 

 may be replied that when the cell is filled with water or with some 

 liquid of higher refractive power, such deflection will in effect be 

 found very small ; and the Author can now say from a large expe- 

 rience that it is practically inoperative. Such cells until recently 

 were costly ; but being now made in large quantities, their price 

 has been so much reduced that they may be obtained more cheaply 

 than cells of any other kind.* For objects whose shape adapts 

 them to the form and depth of the concavity, these cells will be 

 found peculiarly advantageous ; since they do not hold air-bubbles 

 so tenaciously as do those with perpendicular walls, and there is 

 no cemented plate or ring to be loosened from its attachment, either 

 by a sudden 'jar,' or by the lapse of time. "When transparent 

 objects are mounted in them, it is important to take care that the 



* They are sold by Messrs. Jackson, Oxford-street, either of round or 

 oval form, Fig. 104, a, b ; and not only ground-out of slides of the usual 

 size (3 in. by 1 in.) and substance, but also hollowed in pieces of plate 

 glass of larger dimensions (c) and much greater thickness. 



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