104 Rayleigh's Address to the British Association. 



with such success that our knowledge of it begins to be compara- 

 ble with that of the parts visible to the eye. Equally important 

 work has been done by Langley, using a refined invention of his 

 own based upon the principle of Siemens' pyrometer. This in- 

 strument measures the actual energy of the radiation, and thus 

 expresses the effects of various parts of the spectrum upon a 

 common scale, independent of the properties of the eye and of 

 sensitive photographic preparations. Interesting results have also 

 been obtained by Becquerel, whose method is founded upon a 

 curious action of the ultra-red rays in enfeebling the light emitted 

 by phosphorescent substances. One of the most startling of 

 Langley's conclusions relates to the influence of the atmosphere 

 in modifying the quality of solar light. By the comparison of 

 observations made through varying thicknesses of air, he shows 

 that the atmospheric absorption tells most upon the light of high 

 refrangibility ; so that, to an eye situated outside the atmosphere, 

 the sun would present a decidedly bluish tint. It would be 

 interesting to compare the experimental numbers with the law of 

 scattering of light by small particles, given some years ago as the 

 result of theory. The demonstration by Langley of the inade- 

 quacy of Cauchy's law of dispersion to represent the relation 

 between the refrangibility and wave-length in the lower part of the 

 spectrum must have an important bearing upon optical theory. 



The investigation of the relation of the visible and ultra-violet 

 spectrum to various forms of matter has occupied the attention 

 of a host of able workers, among whom none have been more 

 successful than my colleagues at Cambridge, Professors Liveing 

 and Dewar. The subject is too large both for the occasion and for 

 the individual, and I must pass it by. But, as more closely 

 related to opt.es proper, I cannot resist recalling to your notice a 

 beautiful application of the idea of Doppler to the discrimination 

 of the origin of certain lines observed in the solar spectrum. If 

 a vibrating body have a general motion of approach or recession 

 the waves emitted from it reach the observer with a frequency 

 which in the first case exceeds, and in the second case falls short 

 of, the real frequency of the vibrations themselves. The con- 

 sequence is that, if a glowing gas be in motion in the line of sight, 

 the spectral lines are thereby displaced from the position that 

 they would occupy were the gas at rest — a principle which, in the 



