ON MOUNTING. 155 



the diatoms from that line the screw is to be 

 screwed down a little so as to prevent the frame 

 sliding up so high, and this gives another line of 

 diatoms for selection, and this can be repeated until 

 the entire slip has been thoroughly explored. In 

 this manner nothing can be overlooked, since the 

 action of the screw ensures the return of the slip 

 to precisely the same spot. Now, the cover on 

 which the diatoms are to be mounted is to be pre- 

 viously prepared by placing upon it a small drop 

 of the cement already referred to, which cement 

 has to be kept moist during the selecting and 

 arranging the diatoms. For this purpose an 

 ordinary glass tube with a bulb is to be procured 

 and mounted in a piece of brass tubing, having a 

 ball and socket joint, and which is to be made to 

 fit into that part of the microscope stand which is 

 occupied by the side reflector when in use. To 

 the shorter end of the glass tube a piece of india- 

 rubber tubing is to be attached, and a small piece 

 of glass tubing, to be used as a mouthpiece, is to be 

 inserted into the other end of the elastic tubing. 



Now this " breathing apparatus " being fixed in 

 its place on the stand of the microscope, the longer 

 end of the glass tube is to be placed about three 

 inches from, and level with, the thin cover on the 

 glass slip, and the frame C C drawn down to the 

 pin L ; now breathe (do not How so as to blow away 

 the diatoms) upon the cover, and it will be found 

 that the moisture from the breath will at once 

 liquefy the cement, and with a bristle mounted on 

 the cedar stick of a camel-hair pencil as already 

 described carefully remove all dirt, if any, from 

 the cover, then from the lower slip select a diatom 



