236 MOUNTING OF OBJECTS. 



only, by the use of the ingenious little instrument (Fig. 63) con- 

 trived by Mr. Shadbolt. This consists of a small slab of ma- 

 hogany, into one end of 



FIG 63. which is fixed a pivot, 



whereon a circular turn- 

 table of brass, about three 

 inches in diameter, is 

 made to rotate easily, a 

 rapid motion being given 



Shadbolt's Turn-table for making Cement-Cells. to it by the application 



of the fore-finger 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 equidis- 

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

 an inch in diameter, traced upon the brass), and being held by 

 the springs with which it is furnished, a camel-hair pencil dipped 

 in the varnish to be used (Brunswick black or Asphalte is the 

 best) is held in the right hand, so that its point comes into con- 

 tact with the glass, a little within the guiding circle just named. 

 The turn-table being then put into rotation with the left hand, 

 a ring of varnish of suitable breadth is made upon the glass ; 

 and if the slide be set aside in a horizontal position, this ring 

 will be found, when dry, to have lost the little inequalities it may 

 have at first presented, and to present a very level surface. If a 

 greater thickness be desired than a single application will con- 

 veniently make, a second layer may be laid on after the first is 

 dry. It is convenient to prepare a number of these cells at once, 

 since, when "the hand is in," they will be made more dexter- 

 ously than when the operation is performed only once; and it 

 will be advantageous to subject them to the warmth of a slightly 

 heated oven, whereby the flattening of their surface will be more 

 completely assured. The Microscopist will find it a matter ot 

 great convenience to have a stock of these cells always by him, 

 ready prepared for use. 



135. Thin Glass Cells. For the reception of objects too thick 

 for varnish cells, but not thicker than ordinary thin glass, it is 

 advantageous to construct cells of glass ; and these may be made 

 in one of two ways, either by grinding down the cross sections 

 of glass tubes ( 137) until they have been reduced to the desired 

 thinness, or by perforating a plate of thin glass with an aperture 

 of the desired size ; and then cementing the ring or the plate to 

 the glass slide with marine glue. The former plan is liable to 

 the objection, that in reducing the glass rings to the desired 

 thinness, they are extremely liable to crack or break, and that 

 their attainable forms are limited. The latter will generally 

 answer very well, if care be taken in the selection of &flat piece 

 of thin glass ; and the perforation, if due precaution be em- 

 ployed, may be made of any size or form that may be desired. 

 For making round cells, the perforated pieces that sometimes re- 



