MKTHOD OF MOUNTING OBJECTS IN FLUID. 289 



thick, and 3| inches in diameter, which are made to turn 

 together, and in opposite directions, by means of a silken cord 



Fig. 182. 



crossed. A handle, b, is attached to one wheel, and a little 

 spring, c, formed like the letter Y, to the other ; this spring 

 is capable of being raised by pressing the tail of the Y. An- 

 other piece of mahogany, d, is attached to the base, so as to 

 surround about one-third of the wheel carrying the spring, 

 and should be sufficiently thick to be slightly elevated above 

 the said spring; this is intended as a support to the hand. The 

 screws or centres on which the wheels move, e e, must not 

 project in the slightest degree beyond the wood. A few cir- 

 cular lines, from an inch, as the extreme, to any smaller size, 

 should be drawn on the wheel with the spring. Having raised 

 the Y spring, by pressing the end, e, place on the wheel a slip 

 of glass of the usual size, and adjust it so that none of the cor- 

 ners project; if the wheel be made of the size named (3| 

 inches), it will then be centred in one direction, and may be 

 readily centred perfectly by means of the largest ring drawn 

 on the wheel. Having dipped a camel's hair pencil into some 

 appropriate cement, hold it to the glass, at the same time 

 causing the latter to revolve, by means of the second wheel 

 with the handle, a very neat circular cell will thus be formed. 

 The best kinds of cement for this purpose are either the 

 asphalte cement, described page 275 of this work, or the 

 japanners' gold size, or a mixture of the two in equal propor- 

 tions. If one coating be not thick enough for the purpose 

 intended, another can be laid on as soon as the first is dry, as 

 there Is no difficulty in centering. In making cells with the 

 asphalte cement, a coating of gold- size should always be laid 

 on at last, in order to prevent its becoming too brittle, and the 

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