884 MR. Ct. a. SCHOTT ON THE REFLECTION AND REFRACTION OF LIGHT. 
negative) reflection, when they are placed in contact with the same third substance. 
This agrees with the conclusion arrived at by Schmidt [loc. cit.), from his experiments 
on various crown and flint-glasses.—July 20.] 
The above experiments are sufficient to test the accuracy of the theory, which 
merely assumes the existence of a film of transition, without entering into the question 
of its origin and constancy ; wdiether it be due to a surface property of the medium— 
a kind of capillary effect—or merely to an adventitious film of dust or of polishing 
powder, is of no consequence as far as the theory is concerned, its existence is 
the crucial hypothesis, and of that existence there can be no doubt. The theory does 
however point to the idea that the film may be, in part at least, of adventitious 
origin. 
This is confirmed by the experiments of Dhude already mentioned, and those 
of Lord Rayleigh on the reflection from pure water surfaces (‘ B. A. Repts.,’ 1891, 
p. 563), who finds that perfectly clean water reflects only toVo of perpendicularly 
polarized light found by Jamin, so that its ellipticity is only about - 3 ^ of Jamix’s 
value. The darkness of the band observed in the analyser at the polarizing angle was 
disturbed by a small trace of olive oil applied to the surface and producing a thin film. 
§ 12. Conclusion. 
We may sum up the results of the preceding discussion as follows :— 
(1.) A rigid elastic solid theory, proceeding on the assumption that the velocity of 
the pressural-wave is much greater than that of the light-wave, will not explain the 
experimental results, whatever be the refractive index for the pressural-wave. 
( 2 .) Lord Kelvin’s contractile ether theory and the electromagnetic theory in 
Hertz’s form, lead to the same equations, containing three independent constants (of 
which two have little effect, except at a distance from the polarizing angle) ; and these 
equations agree with the experiment ratlier better than Cauchy’s empirical formulae 
containing, as used by Jamin, one constant, e, and as used by Quincke, two con¬ 
stants, e and /r'. At a distance from the polarizing angle Fresnel’s expression for 
the intensity is sufficient. 
(3.) Whilst Cauchy’s constants, e, have been found not to satisfy the theoretical 
conditions assigned by Jamin (so that Cauchy’s formula must be regarded as an 
empirical one), the constants of the above theoretical formulae satisfy the conditions 
theoretically deduced, as nearly as is to be expected, considering that the whole effect 
under discussion is itself but a small correction. 
This last conclusion as to the possibility of a theoretical explanation of the 
j^henomena of reflection based on the existence of a film of transition is at variance 
with the result arrived at by M. H. Bouasse from a critical examination of the 
theories so far proposed. (See his paper in the ‘ Annales de Chimie et Physique ’ for 
February, 1893, p. 145.) 
