INTRODUCTION. 43 



by those who possess an achromatic microscope ; but with a 

 simple instrument the triplet can be used only when talc is 

 the covering. 



Specimens are frequently spoiled by the intrusion of the 

 gold-size into the cell. Experience will best teach how this 

 accident is to be avoided ; attention however to the following 

 particulars will assist the inexperienced. The cells should 

 be prepared some days before they are used, in order that 

 their walls may become firm. When the cell is closed the 

 brush should be passed round the edge of the cover, with just 

 sufficient size to prevent the admission of air into the cell ; 

 and upon the operator's care in this respect will depend his 

 success. If too little size be used the air finds admission, 

 and if much be put on, or if the cell be not completely filled 

 with fluid, the size will enter and spoil the specimen. When 

 the first layer of size is quite dry, he should proceed as di- 

 rected by Mr. Thwaites. 



Mr. Topping has kindly sent me a description of his 

 method of preparing cells for mounting microscopic objects. 

 He uses strips of plate-glass of an uniform size (three inches 

 by one), and marks on them the size of the cell, by taking 

 two thin pieces of mahogany of the size of the glass, each 

 having a hole (circular, oval or square, as may be desired) 

 cut in its centre, the smaller corresponding with the inner 

 margin of the cell, and the larger with the outer. These, 

 when laid over the glass, offer a ready means of tracing with 

 a diamond the space around the cell, which must then be 

 filled up with japan. This is next hardened by placing the 

 glasses in an oven, the heat of which should be raised gra- 

 dually, as otherwise the japan will blister ; but if care be 

 taken in this part of the process the cells will resist the action 

 of proof spirit. 



The fluid which Mr. Topping has used for mounting con- 

 sists of one ounce of rectified spirit to five ounces of distilled 

 water, which he thinks superior to any other combination. 

 To preserve delicate colours however, he prefers to use a so- 

 lution of acetate of alumina — one ounce of the acetate to four 



