The object of polishing is to produce regular transparent surfaces on a piece of glass or other clear substance. The process is divided into two: grinding and polishing. They are commonly held to be quite different in character, although this opinion is not universal. Lord Rayleigh states that the particles of emery in grinding glasses appear to act by pitting the glasses. In one experiment a glass surface was polished with the same emery as had been used effectively a little earlier in the grinding. The difference is in the character of the backing. In grinding the emery is backed by a hard surface, e.g. of glass, while during the polishing the powder is imbedded in a comparatively yielding substance, such as pitch. The nature of the ground glass surfaces was studied very thoroughly by Dr F. W. Preston. The practical optician Rayleigh, in polishing glass with rouge on pitch, finds that if the pitch is too hard he scratches.