METHOD OF MOtlNTING OBJECTS IN FLUID. 291 



horizontal position, to prevent more of the glue getting on one 

 side than the other. When allowed to cool, but before 

 becoming quite hard, a piece of cold glass should be pressed on 

 the top of each ring, and left there till cold ; this is to cause 

 the ring to be quite flat at top, ready for the reception of the 

 cover. On pulling the glasses apart, it will be found that the 

 glue will readily separate from the glass that was cold, and 

 remain firmly fixed to the other. The covers can be cemented 

 on when- the cells are filled, by means of the apparatus pre- 

 viously described. 



The Thin Glass Cell. — The cell represented by fig. 183 was 

 first employed by Mr. Goadby, and consists, as its name 

 implies, of a piece of thin glass, such as is 

 used for covers, about three-quarters of an 

 inch square, out of which a round hole, 

 varying from one-half to five-eighths of an 

 inch, has been drilled. In order to make 

 this useful, it is to be cemented to one of 

 j,.^ the shdes of plate-glass with marine-glue, 



in the manner previously described at page 

 271. After the cell has been properly cleaned, as there 

 directed, it is ready to receive the intended object, which is to 

 be mounted as follows: — A small quantity of gold-size is to be 

 placed upon the margins of the ceU, and as much as possible 

 wiped off with the finger (taking care that the size is wiped 

 away from the hole of the cell and not towards it) ; the fluid 

 in which the object is to be mounted must now be placed in 

 the cell (and it is always a good plan to put in rather more 

 than is used), and after the object has been properly arranged, 

 the cover previously cleaned and anointed on its edges, as in 

 the case of the flat cell, is to be laid on the fluid and pressed 

 down, and the excess removed from its edges by the blotting 

 paper or sucking tube, and the cement laid on in the manner 

 before described. When these cells are not sufiiciently thin, 

 they may be readily made so by rubbing them down on the 

 metal plate described in page 264, with some fine emery and 

 water. It is a good plan always to give them a rub on the 

 metal, not only in order that they may be rendered flat, 

 19* 



