16 EEPORT — 1884. 



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 out- 

 side 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 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 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 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 atTnospheric 

 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 appears to tremble, as the result 

 of slight alternately opposite displacements. But if the seat of the ab- 

 sorption be in the atmosphere, it is a matter of indifference from what 

 part of the sun the light originally proceeds, and the line maintains its 

 position in spite of the oscillation of the image upon the slit of the spec- 

 troscope. In this way Cornu was able to make a discrimination which 



