80 MAKING AND MOUNTING 



turn at every rub ; a very little practice will enable anyone to 

 make the discs almost circular. But what is chiefly to be aimed 

 at in making them circular, is to get a smooth edge, as a disc having 

 a perfectly smooth edge will not break so readily in the subsequent 

 process, as a rough-edged one. It should now measure about 

 five-eighths of an inch in diameter. The flat face must next be 

 polished, so as to remove every trace of scratching caused by the 

 sandstone, and at the same time it is necessary to make this face 

 perfectly flat. To accomplish this, I use a Water-of-Ayr hone, 

 seven inches square, by two and a half inches thick, having one of 

 the ia.Q,QS perfectly flat. On this face the disc is rubbed with water, 

 until it also becomes perfectly flat and free from scratches. It 

 must then be made thoroughly clean and 

 mounted on a piece of hard wood, as shown 

 in Fig. 3. 



I use well-seasoned Beech wood, two 

 inches square by three quarters of an inch 

 thick. Fix the disc to the block of beech 

 with gum arable, putting plenty of the gum 

 round the sides, so as to form a collar, and 

 allow it to harden for two or three days. 

 The specimen is now to be ground down, until the beech can 

 be seen distinctly through it. It will not do to rub it on the sand- 

 stone now, as water would dissolve the gum, and the specimen would 

 be at once detached. For the purpose of rubbing it down, use a flat 

 metal plate, coarse emery powder, and parafifine, turpentine, or 

 benzoline, as none of these substances will dissolve gum arable. 

 Mr. Pratt used to rub his specimens down entirely on the metal 

 plate, using finer emery powder as the specimen became thinner. 

 But after it has been reduced to about the twentieth of an inch in 

 thickness, I find I can make more speed by using a Turkish 

 whetstone sprinkled with a Uttle of the finest emery powder, and 

 rubbing on this till the wood may be dhnly seen through the 

 specimen. At this stage I clean the specimen, wood, and 

 whetstone, with a piece of rag soaked in turpentine, and rub down 

 on the bare stone, using the same fluid, till the specimen is thin 

 enough to be taken off the wood. Of course, this is the 

 most critical period in the rubbing process, and the period 



