752 THE POPULAR SCIENCE MONTHLY. 



ley, using a refined invention of his own based upon the principle of 

 Siemens's pyrometer. This instrument 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 ab- 

 sorption tells most upon the light of high refrangibility ; so that, to 

 an eye situated outside the atmosphere, the sun would present a de- 

 cidedly bluish tint. It would be interesting to compare the experi- 

 mental 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 inadequacy of Cauchy's law of dispersion to represent 

 the relation between 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 optics proper, 

 I can not resist recalling to your notice a beaiitiful application of the 

 idea of DopjDler 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 them- 

 selves. The consequence 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 hands of Huggins and others, has led to a determination of the 

 motion of certain fixed stars relatively to the solar system. But the 

 sun is itself in rotation, and thus the position of a solar spectral line 

 is slightly different according as the light comes from the advancing 

 or from the retreating limb. This displacement was, I believe, first 

 observed by Thollon ; but what I desire now to draw attention to is 

 the application of it by Cornu to determine whether a line is of solar 

 or atmospheric origin. For this purpose a small image of the sun is 

 thrown upon the slit of the spectroscope, and caused to vibrate two 

 or three times a second, in such a manner that the light entering the 

 instrument comes alternately from the advancing and retreating limbs. 

 Under these circumstances a line due to absorption within the sun 



