18 Lord Rayleigh on Reflexion from Liquid Surfaces, 



ellipticity is abundantly evident. For example, camphor 

 was quite active upon a surface which gave a difference of 

 readings of —80', corresponding to k= — "007. On this 

 occasion the surface had stood for some time without much 

 protection, and it is possible that the effect may have been 

 partly due to dust. 



The last example that I will mention of aqueous solutions is 

 a strong brine. This gave a somewhat variable difference of 

 about —5', corresponding to h= —'00042. In this case there 

 seemed to be unusual difficulty in getting the surface clean, 

 so that the difference between the brine and pure water is 

 not improbably due to some secondary cause. 



Most of the available time was spent upon water in its 

 various states, not only on account of its intrinsic importance, 

 but also because of the presumably greater simplicity of a 

 clean water-surface. The observations are made in an atmo- 

 sphere which contains no very small proportion of aqueous 

 vapour. When the liquid under examination has an affinity 

 for water, e. g. alcohol, it is difficult to form a precise idea as to 

 what may be the condition of the surface. Besides, the 

 arrangements for skimming are less easily applied. On the 

 other hand, the liquids of lower tension are less likely to 

 acquire a film of grease. For alcohol, and also for petroleum, 

 the value of k is about +'0010. 



The general conclusion to be drawn from these investiga- 

 tions is that the ellipticity of the liquids examined is very 

 much less than was supposed by Jamin, whose results for 

 water and aqueous solutions were almost certainly vitiated by 

 the presence of greasy contamination. Thus the intensity of 

 reflexion from clean water is not much more than jqVo P ar ^ 

 of that given by Jamin. Moreover, the value of k is posi- 

 tive, and not negative. It is even possible that there would 

 be no sensible ellipticity for the surface of a chemically pure 

 body in contact only with its own vapour. But the surfaces 

 of bodies are the field of very powerful forces of whose action 

 we know but little ; and even if there be nothing that could 

 be called chemical change, the mere want of abruptness in 

 the transition would of itself entail a complication. There is 

 thus no experimental evidence against the rigorous applica- 

 bility of Fresnel's formulae to the ideal case of an abrupt 

 transition between two uniform transparent media. 



September 19. 



