228 SELECTION AND USE 



mended very highly, and seem to answer a very good purpose. 

 The cells are not bored quite through the wooden slip, and as 

 they are blackened on the inside, any small object that may be 

 cemented to the bottom of them shows very well. For seeds, 

 small shells, and similar objects, they answer admirably. In 

 most cases it will be found unnecessary to cover the cells with 

 thin glass. Several slides may be packed together face to face, 

 and if held in firm contact by means of a rubber ring, dust will 

 be entirely excluded. Or they may be arranged in the drawers 

 of an ordinary cabinet, face down, the labels being placed on 

 the backs. This will effectually exclude the dust. 



Some years ago we mounted a large number of specimens of 

 minerals on leather discs, which were cemented to glass slides. 

 These leather discs were three-quarters of an inch in diameter, 

 and we had a lot of pill-box covers which exactly fitted them. 

 These covers, when slipped on to the discs, protected the ob- 

 jects perfectly, and the whole formed a very cheap, convenient 

 and excellent mode of mounting. 



A very ingenious cell for opaque objects, the invention of 

 Prof Pierce, of Provi- 

 dence, R. I. , is shown in 

 Fig. 75. It consists of a 

 metallic cell, having a 

 broad flange like the 

 rim of a hat, which is 

 cemented to an ordinary Fig. 75. 



glass slide, as shown in 



section in the lower figure. To this cell is fitted a metal cap, 

 which covers and protects the object. The object may be placed 

 directly on the glass, or raised by means of a disc of any re- 

 quired thickness, so as to be more easily illuminated. The 

 slide, with cell uncovered and containing an object, is shown 

 in the upper figure. Uncovered objects may in this way be 

 very perfectly protected from dust and mechanical violence. 



Some persons object to any slide that is mounted without a 

 glass cover. It must be acknowledged, however, that while 

 glass covers add to the appearance of the object and serve to 

 protect it, they interfere somewhat with its examination, as it 

 cannot be so brilliantly illuminated, and the rays in their passage 



