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



unity, the ellipticity should be slightly increased and the 

 phase difference markedly decreased by the presence of such 

 a film or layer. Expressed in terms of the principal incidence 

 and azimuth instead of the phase difference and ellipticity, 

 the theory predicts that the effect of the film should be to 

 decrease the principal incidence markedly and to increase 

 the principal azimuth but a very little. Unlike the result 

 for the reflexion from transparent substances, the sign of the 

 effect is independent of the magnitude of the index of 

 refraction of the reflecting medium, unless that should be 

 less than unity. 



We have here, then, no such criterion as before to enable 

 us to distinguish between films of surface contamination and 

 a real transition layer. A possible exception to this state- 

 ment is the case of those metals which have an index of 

 refraction less than unity. That some metals, notably gold, 

 silver, and copper, do possess such small values of the index 

 seems certain, since the same result is yielded by the reflexion 

 and the direct transmission methods. With these substances 

 it might be expected that the transition layer would have an 

 index less than unity, and that consequently, according to 

 the theory, the phase difference would be increased and the 

 ellipticity decreased over the values they would have if no 

 such layer were present. However, from general consider- 

 ations as to the degree to which it would be possible for the 

 two media to interpenetrate and produce a transition layer, 

 it would seem unlikely that we could obtain any evidence as 

 to its existence ; exactly as any such evidence fails in the 

 case of reflexion from transparent solids. And in confirm- 

 ation of this expectation it is found that for all metals, 

 increasing the cleanliness of the surface shows uniformly an 

 increase of the phase difference and a decrease of the 

 ellipticity ; there is never any indication of any reversal of 

 the sign of the effect. So that it is fair to assume that when 

 perfect cleanliness has been attained, the correct values of 

 the phase difference and the ellipticity are yielded by 

 experiment ; the effect of the transition layer being negligible. 



Further light on the question of the transition layer is 

 afforded by experiments on the reflexion from metal mirrors 

 immersed in transparent liquids. Here the uncorrected 

 theory leads to an expression for the index of refraction of 

 the liquid (wQ in terms of the observed phase differences and 

 ellipticities in air and in the liquid respectively *. An 

 examination of the recorded observations of this nature shows, 

 however, very considerable discrepancies between the values 

 * See below, under heading " Theory." 



