Rayleigh's Address to the British Association. 105 



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 

 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 spectro- 

 scope, 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 circum- 

 stances 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 absorption 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 spectroscope. In this way Cornu 

 was able to make a discrimination which can only otherwise be 

 effected by a difficult comparison of appearances under various 

 solar altitudes. 



The instrumental weapon of investigation, the spectroscope 

 itself, has made important advances on the theoretical side. We 

 have for our guidance the law that the optical power in gratings is 

 proportional to the total number of lines accurately ruled, without 

 regard to the degree of closeness, and in prisms that it is propor- 

 tional to the thickness of glass traversed. The magnificent 

 gratings of Rowland are a new power in the hands of the spectro- 

 scopist, and as triumphs of mechanical art seem to be little short 

 of perfection. In our own report for 1882 Mr. Mallock has 

 described a machine constructed by him, for ruling large diffrac- 

 tion-gratings, similar in some respects to that of Rowland. 



The great optical constant, the velocity of light, has been the 

 subject of three distinct investigations by Cornu, Michelson, and 

 Forbes. As may be supposed, the matter is of no ordinary 

 difficulty, and it is therefore not surprising that the agreement 

 should be less decided than could be wished. From their obser- 

 vations, which were made by a modification of Fizeau's method 

 of the toothed wheel, Young and Forbes drew the conclusion that 



