1901.] on Polish. 567 



than Herschel thought between the surfaces of a polished solid and 

 of a liquid. 



Several trials have been made to determine how much material 

 is actually removed during the polishing of glass. In one experiment 

 a piece 6 inches in diameter, very finely ground, was carefully weighed 

 at intervals during the process. Losses of "070, "032, "045, "026, 

 •032 gms. were successively registered, amounting in all to *205 gms. 

 Taking the specific gravity of the glass as 3, this corresponds to a 

 thickness of 3*6 X 10 — * cm., or to about 6 wave-lengths of mean 

 light, and it expresses the distance between the original mean surface 

 and the final plane. But the polish of this glass, though sufficient 

 for most practical purposes, was by no means perfect. Probably the 

 6 wave-lengths would have needed to be raised to 10 in order to 

 satisfy a critical eye. It may be interesting to note for comparison 

 that, in the grinding, one charge of emery, such as had remained 

 suspended in water for seven or eight minutes, removed a thickness 

 of glass corresponding to 2 wave-lengths. 



In other experiments the thickness removed in polishing was de- 

 termined optically. A very finely ground disc was mounted in the 

 lathe and polished locally in rings. Much care was needed to obtain 

 the desired effect of a ring showing a continuously increasing polish 

 from the edges inwards. To this end it was necessary to keep the 

 polisher (a piece of wood covered with resin and rouge) in constant 

 motion, otherwise a number of narrow grooves developed themselves. 



The best ring was about half an inch wide. When brought into 

 contact with a polished flat and examined at perpendicular incidence 

 with light from a soda flame, the depression at its deepest part gave a 

 displacement of three bands, corresponding to a depth of 1^ A. On 

 a casual inspection this central part appeared well polished, but ex- 

 amination under the microscope revealed a fair number of small pits. 

 Further working increased the maximum depth to 2^ A, when but 

 very few pits remained. In this case, then, polish was effected during 

 a lowering of the mean surface through 2 or 3 wave-lengths, but 

 the grinding had been exceptionally fine. 



It may be well to emphasise that the observations here recorded 

 relate to a hard substance. In the polishing of a soft subtance, such 

 as copper, it is possible that material may be loosened from its original 

 position without becoming detached. In such a case pits may be 

 actually filled in, by which the operation would be much quickened. 

 Nothing suggestive of this effect has been observed in experiments 

 upon glass. 



Another method of operating upon glass is by means of hydro- 

 fluoric acid. Contrary to what is generally supposed, this action is 

 extremely regular, if proper precautions are taken. The acid should 

 be weak, say one part of commercial acid to two hundred of water, 

 and it should be kept in constant motion by a suitable rocking 

 arrangement. The parts of the glass not intended to be eaten into 

 are, as usual, protected with wax. The effect upon a polished flat 



