564 TJie Bight Hon. Lord Bayleigh [March 29, 



few residual pittings, although they spoil the appearance of a surface, 

 do not interfere with its performance, at least for many purposes. 



In the process of grinding together two glass surfaces, the particles 

 of emery, even the finest, appear to act by pitting the glasses, i.e. by 

 breaking out small fragments. In order to save time and loss of 

 accuracy in the polishing, it is desirable to carry the grinding process 

 as far as possible, using towards the close only the finest emery. The 

 limit in this direction appears to depend upon the tendency of the 

 glasses (6 inches diameter) to seize, when they approach too closely, 

 but with a little care it is easy to attain such a fineness that a candle 

 is seen reflected at an angle of incidence not exceeding 60°, measured 

 as usual from the perpendicular. 



The fineness necessary, in order that a surface may reflect and 

 refract regularly without diffusion, viz. in order that it may appear 

 polished, depends upon the wave-length of the light and upon the 

 angle of incidence. At a grazing incidence all surfaces behave as if 

 polished, and a surface which reflects red light pretty well may fail 

 signally when tested with blue light at the same angle. If we consider 

 incidences not too far removed from the perpendicular, the theory of 

 gratings teaches that a regularly corrugated surface behaves as if 

 absolutely plane, provided that the wave-length of the corrugations is 

 less than the wave-length of the light, and this without regard to the 

 depth of the corrugations. Experimental illustrations, drawn from 

 the sister science of Acoustics, were given. The source was a bird- 

 call from which issued vibrations having a wave-length of about 

 1 • 5 cm., and the percipient was a high-pressure sensitive flame. 

 When the bird-call was turned away, the flame was silent, but it 

 roared vigorously when the vibrations were reflected back upon it 

 from a plate of glass. A second plate, upon which small pebbles had 

 been glued so as to constitute an ideally rough surface, acted nearly 

 as well, and so did a piece of tin plate suitably corrugated. In all 

 these cases the reflection was regular, the flame becoming quiet when 

 the plates were turned out of adjustment through a very small angle. 

 In another method of experimenting the incidence was absolutely per- 

 pendicular, the flame being exposed to both the incident and the 

 reflected waves. It is known that under these circumstances the 

 flame remains quiescent at the nodes and flares most vigorously at 

 the loops. As the reflector is drawn slowly back, the flame passes 

 alternately through the nodes and loops, thus executing a cycle of 

 changes as the reflector moves through half a wave-length. The 

 effects observed were just the same whether the reflector were smooth 

 or covered with pebbles, or whether the corrugated tin plate were 

 substituted. All surfaces were smooth enough in relation to the wave- 

 length of the vibration to give substantially a specular reflexion. 



Finely-ground surfaces are still too coarse for perpendicular 

 specular reflexion of the longest visible waves of light. Here the 

 material may be metal, or glass silvered chemieally on the face sub- 

 sequently to the grinding. But experiment is not limited by the 



