A careful comparison of theory with experiment as regards the intensity of reflection would seem to suggest itself naturally as a crucial test of the validity of any optical theory. In spite of this, it was not till late in the last century that the problem was seriously undertaken by experimentalists. In 1870 Rood turned his attention to. the subject with the view of testing Fresnel’s laws, and concluded from his experiments “that the reflecting power of glass conforms, in the closest manner, to the predictions of theory.” However, in 1886, this conclusion was shown to be untenable by Lord Rayleigh. The difficulties of measuring the intensity of the reflected light accurately are very considerable, and Rood had contented himself with estimating the transmitted light and deducing the amount that was reflected. Rayleigh showed that when this fact was considered the difference between Fresnel’s formula and Rood’s experimental results might amount to 7 per cent, of the reflected light, a difference much too great to be regarded as insignificant. Rayleigh found from his own experiments that recently polished glass has a reflecting power differing not more than 1 or 2 per cent, from Fresnel’s formula; but that after some months or years the reflection may fall off 10 per cent, or more, and that without any apparent tarnish. About the same time Sir John Conroy carried out a lengthy series of experiments on the same subject. His results were published in the ‘Phil. Trans.,’ 1888, and confirmed those of Lord Rayleigh. There can thus be no doubt of a decided departure from Fresnel’s formula under certain circumstances. The difference is too great to be put down to experimental errors, and there is no evidence of such errors, seeing that the results of experiment are fairly consistent. Nor can there be very much doubt as to the direction in which to look for an explanation of the apparent divergence between theory and experiment. Everything points to a changing condition of the reflecting surface, and this suggests that a consideration of the layer of transition will show how Fresnel’s laws are departed from in this as in some other directions. The object of the present paper is to investigate this matter rather more systematically than appears to have been done hitherto.
On the intensity of light reflected from transparent substances
