THE MICROSCOPE. 23 



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 faces 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. 



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 arabic, 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, course emery powder, and paraffine, turpen- 

 tine, or benzoline, as none of these substances will dissolve gum 

 arabic. 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 little of the finest emery powder, and 

 rubbing on this till the wood may be dimly 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 

 at which one is most apt to lose his temper, for if the stone is a 

 hard one, progress is very slow; the rubbing must be done very 

 gently. If the digestion is at all out of sorts, the destruction of 

 the specimen is all but certain, and if a single grain of emery lin- 

 gers about the whetstone, the thin and transparent section will 

 inevitably be cut in two. Lately, I have fallen on a plan which has 

 proved an almost perfect cure for impatience and ill nature, at this 

 critical stage of the proceedings. It is this: Keep a dozen speci- 

 mens advancing one after the other. When you have become tired 



