The Microscope. 85 



to be used with fluid or with balsam mounts, the air in the hollow 

 of the tubular ring will in time ooze into the mount, and perhaps 

 lift the cover, or do some other damage. 



PAPER RINGS. 



Another kind sometimes used and capable of being made at 

 home are rings of paper soaked in shellac, and dried. Paper of 

 any desired thickness is cut into disks, preferably with a gun 

 wad punch, and made into rings by cutting out the centres with 

 a punch of another size. These rings are then soaked in thin 

 shellac until thoroughly saturated. When dry they can be kept 

 for any length of time, and fastened to the slip by being gently 

 heated or by being wet by a drop of shellac. With care, good 

 rings may be made very cheaply in this way, and sightly mounts 

 prepared with them, but they are brittle and must be handled 

 with caution. 



HARD RUBBER RINGS. 



Vulcanite or hard rubber rings must be bought of the dealers, 

 if desired. They are better than tin only in being lighter, a thick 

 rubber ring being less weighty than a thin tin one. I do not 

 know that they have any other advantage, except perhaps in 

 price. Like the other kinds they can be had in various thick- 

 nesses and diameters. 



THE ATWOOD RUBBER CELL. 



The Atwood rubber cell is a solid rubber disk, with a concave 

 upper surface, in which the object is placed. Around the edge 

 is a depression to receive the cover glass, and the whole is ce- 

 mented to the slip. These cells are intended only for opaque 

 objects, that is, for those which must be examined with the 

 microscope when the light is reflected down on them from the 

 mirror rotated above the stage, or by the bull's eye condenser. 

 Their use is therefore rather limited. For their special purpose, 

 however, they may be commended, although there are other 

 ways of mounting opaque objects quite as good. The cells seem 

 to be made to receive only a half-inch cover glass. 



THE PIERCE CELL. 



Another cell somewhat similar to the Atwood, but made from 

 brass, and with a cap-like cover, is occasionally seen; but I do 

 not know that it is in the market. It is the Pierce cell, and con- 



