234 Prof. L. P. Wheeler on the Reflexion of 



by Drude *, this result must be interpreted as due to a film 

 or layer having an index less than that of the liquid. This 

 suggests that the effect is due to a film of air or other gas 

 caught between the liquid and the mirror, since the transition 

 layer of a liquid-air surface would have an index less than 

 that of the liquid. It is, moreover, natural to expect the 

 presence of such films due either to entrainment in the act 

 of immersion or to dissolved gas in the liquid, more probably 

 the former. 



On the other hand, if it is assumed that no such gaseous 

 film is present, the observed discrepancies must be attributed 

 1o a transition layer due to the liquid. In view of the fact 

 that the effects of the transition layer in the case of trans- 

 parent solids in air are inappreciable, it would seem that the 

 layer in question here cannot be a region of interpenetration 

 of the metal and the liquid. Consequently we must conclude 

 (if we reject the possibility of the gaseous film) that the 

 observed discrepancies are due to a layer at the surface of 

 the liquid which is independent of the medium with which 

 it may be in contact and which has optical properties 

 different from those in the interior. According to the theory, 

 the index of such a layer will be less than that of the interior 

 if n' is less than n ; while its index should be greater when 

 n' is greater than n Q . Such effects might possibly be expected 

 as a result of surface tension. 



There are thus two possible explanations of the observed 

 discrepancies. It would seem worth while, therefore, to 

 eliminate the possibility of gaseous films between the mirror 

 and the liquid. The desirability of doing this has been 

 pointed out by Drude t, but so far as I can ascertain, it has 

 not, up to the present time, been attempted. Drude has 

 expressed the opinion that there would still remain evidence 

 of a transition layer when such gaseous films have been 

 removed J. But this opinion is supported by no evidence 

 which cannot be interpreted, as we have seen, in another 

 manner. Hence I have attempted in the investigation 

 reported in this paper to see if any residual effect of a 

 transition layer remains, when the possibility of the presence 

 of a gaseous film between the mirror and the liquid is reduced 

 to a minimum. 



* Wied. Ann. xxxix. 1891, p. 539. See also below, under heading 

 " Theory." 



t Wied. Ann. xxxix. 1891, p. 545. Also Winkelmann, Handbach der 

 Physik, 2 te Aufl. 1906, vol. vi. p. 1308. 



\ Winkelrnann, he. ait. 



