275 
1912-13.] The Scattering of Light. 
W. Rosenhain entitled “ On Abrasives and Polishing Materials.” * Briefly 
put, the theory is as follows. The abrading material must be regarded 
as a collection of sharp points and edges, so that the areas of contact 
between the abrasive and the glass are very minute. Hence the resulting 
pressure per unit area is in many cases greater than the tensile strength of 
the glass, with the result that as the grinding progresses conchoidal 
fractures take place all over the surface of the latter. Grinding is to be 
sharply distinguished from polishing, inasmuch as polishing produces 
14. Calculated case of semicylindrical cavity towards light. 
15. Calculated case of hemispherical cavity towards light. 
A. Cavity equivalent to Case no. 1. 
B. Cavity equivalent to Case no. 8. 
C. Cavity which gives rise to uniform intensity. 
Curves of relative intensity. Curve of maximum total emission. 
“ surface flow,” j- while grinding does not. In the present connection the 
application of the fact lies in this, that it follows that all the myriad 
angles of the ground glass are perfectly sharp, and free from any rounding 
off. Unfortunately, so far as I am aware, no information is available as to 
the exact shapes of the pits produced by grinding. I hope later on to be 
* Trans. Opt. Soc., vol. xi. p. 113, 1909-10. 
t Lord Rayleigh could find no evidence of surface flow in glass ( loc . cit.), but other 
experimenters are satisfied of its existence (see “discussion’’ at end of Lord Rayleigh’s 
paper) ; and it is well established as regards a great many other materials. See especi- 
ally the Hurter Memorial Lecture given by G. Beilby, on “The Surface Structure of 
Solids,” Jour. Soc. Chem. Ind., p. 1166, 1903. 
