﻿432 Lord Rayleigh on Departures from 



which left the flame still easily visible. Rotation of the 

 diamond surface in its own plane seemed to be without 

 effect. 



During the last few months I have resumed these observa- 

 tions, using the same diamonds, but with such additions to 

 the apparatus as are necessary for obtaining measures of the 

 residual reflexion. Besides the polarizing nicol, there is 

 required a quarter- wave mica plate and an analysing nicol, 

 to be traversed successively by the light after reflexion, as 

 described in my former papers. The analysing nicol is set 

 alternately at angles /S= +45°. At each of these angles 

 extinction may be obtained by a suitable rotation of the 

 polarizing nicol ; and the observation consists in determining 

 the angle a! — a between the two positions. Jamin's k, 

 representing the ratio of reflected amplitudes for the two 

 principal planes when light incident at the angle tan -1 //, is 

 polarized at 45° to these planes, is equal to taiiy(a' — a). 

 The sign of a' — a is reversed when the mica is rotated through 

 a right-angle, and the absolute sign of k must be found 

 independently. 



Wiped with an ordinarily clean cloth, the diamond gave 

 at first a — a = 2°'3. By various treatments this angle could 

 be much reduced. There was no difficulty in getting down 

 to 1°. On the whole the best results were obtained when 

 the surface was finally wiped, or rather pressed repeatedly, 

 upon sheet asbestos which had been ignited a few minutes 

 earlier in the blowpipe flame ; but the}" were not very 

 consistent. The lowest reading was 0°*4 ; and we may, I 

 think, conclude that with a clean surface a.' — a would not 

 exceed 0°*5. No more than in the case of glass, did the 

 effect seem sensitive to moisture, no appreciable difference 

 being observable when chemically dried air played upon the 

 surface. It is impossible to attain absolute certainty, but my 

 impression is that the angle cannot be reduced much further. 

 So long as it exceeds a few tenths of a degree, the paraffin 

 flame is quite adequate as a source of light. 



If we take for diamond a! — a = 30', we get 

 * = tan !(*'-«) = -0044. 



Jamin's value for k is "019, corresponding more nearfy with 

 what I found for a merely wiped surface. 



Similar observations have been made upon the face of a 

 small dispersing prism which has been in my possession some 

 45 years. When first examined, it gave x — a — 9°, or there- 

 abouts. Treatment with rouge on a piece of calico, stretched 

 over a glass plate, soon reduced the angle to 4° or 3°. but 



