﻿FresneVs Laws of Reflexion. 433 



farther progress seemed more difficult. Comparisons wero 

 rendered somewhat uncertain by the fact that different parts 

 of the surface gave varying numbers. After a good deal of 

 rubbing, ex.' — a. was reduced to such figures as 2°, on one 

 occasion apparently to 1\°. Sometimes the readings were 

 taken without touching the surface after removal from the 

 rouge, at others the face was breathed upon and wiped. In 

 general, the latter treatment seemed to increase the angle. 

 Strong sulphuric acid was also tried, but without advantage, 

 as also putty-powder in place of or in addition to rouge, 

 The behaviour did not appear to be sensitive to moisture, or 

 to alter appreciably when the surface stood for a few days 

 after treatment. 



Thinking that possibly changes due to atmospheric in^ 

 fluences might in nearly half a century have penetrated 

 somewhat deeply into the glass, I re-ground and polished 

 (sufficiently for the purpose) one of the originally unpolished 

 faces of the prism, but failed even with this surface to reduce 

 a.' — a below 2°. As in the case of the diamond, it is im- 

 possible to prove absolutely that a' — a cannot be reduced to 

 zero, but after repeated trials I had to despair of doing so. 

 It may be well to record that the refractive index of the glass 

 for yellow rays is l'G80. 



These results, in which k (presumably positive) remained 

 large in spite of all treatment, contrast remarkably with those 

 formerly obtained on less refractive glasses, one of which, 

 however, appeared to contain lead. It was then found that 

 by re-polishing it was possible to carry k down to zero and 

 to the negative side, somewhat as in the observations upon 

 water it was possibly to convert the negative h of ordinary 

 (greasy) water into one with a small positive value, when the 

 surface was purified to the utmost. 



There is another departure from FresnePs laws which is 

 observed when a piece of plate glass is immersed in a liquid 

 of equal index *. Under such circumstances the reflexion, 

 ought to vanish. 



The liquid may consist of benzole and bisulphide of carbon, 

 of which the first is less and the second more refractive than the 

 glass. If the adjustment is for the yellow, more benzole or a 

 higher temperature will take the ray of equal index towards 

 the blue and vice versa. li For a closer examination the plate 

 was roughened behind (to destroy the second reflexion), and 



* " On the Existence of Reflexion when the relative Refractive Index 

 is Unity," Brit. Assoc. Report, p, 583 (1887) ; Scientific Papers, vol. iii. 

 p, 15. 



PJiiK Mag. S. G. Vol. 23. No. 135, March 1912. % G 



