MICROSCOPIC OBJECTS. 137 



employ one about fifteen inches in diameter. This plate 

 or wheel must be attached to an axis, and made to revolve 

 rapidly. When ready for use about half an inch around 

 the circumference on both sides is to be paved with 

 diamond powder. This is done by rubbing the diamond 

 powder, previously mixed in ohve oil, on to the plate 

 with a hardened steel roUer. The wheel is now ready 

 for use, and the fossil is to be held firmly against its 

 edge while the wheel is rapidly revohing. In this way 

 a thin slice is cut off the fossil. This shce is then to be 

 cemented to a convenient block of metal, and the other 

 side ground upon a flat metal tool with emery-pow^der 

 and water. When this process is completed, the surface 

 must be smoothed by grinding on another flat tool of 

 slate, after which it is to be pohshed. The process of 

 polishing may be performed in various ways. The best 

 is to procure two discs of plate glass about five inches 

 diameter; warm one of the plates, and pour upon it (as 

 evenly as possible) a thin layer of pitch. When this is 

 cool, take some opticians' putty-powder (oxide of tin and 

 lead), mix it in a phial mth cold spring water, and pour 

 a little over the pitch ; then rub the other plate of glass 

 upon it, so as to render the surface of the pitch perfectly 

 flat, without however allowing the upper plate of glass to 

 adhere to the pitch. If the surface of tha pitch cannot 

 be readily made flat, the upper plate must be left on and 

 pressed down by a heavy weight. 



When the pohsher is prepared, the fossil is to be 

 lightly rubbed upon the pitch, using the putty-powder 



