56 Royal Society : — 



of the stars relatively to our system. If the stars were moving to- 

 wards or from the earth, their motion, compounded with the earth's 

 motion, would alter to an observer on the earth the refrangibility of 

 the light emitted by them, and consequently the lines of terrestrial 

 substances would no longer coincide in position in the spectrum with 

 the dark lines produced by the absorption of the vapours of the 

 same substances existing in the stars. 



The method employed by them would certainly have revealed an 

 alteration of refrangibility as great as that which separates the lines D. 

 They had, therefore, proof that the stars which they had examined, 

 among others Aldebaran, a Orionis, Pegasi, Sirius, a Lyrae, Ca- 

 pella, Arcturus, Castor, Pollux, were not moving with a velocity 

 which would be indicated by such an amount of alteration of position 

 in a line. 



Since, however, a change of refrangibility corresponding to that 

 which separates the components of D would require a velocity of 

 about 190 miles per second, it seemed to them premature to refer to 

 this bearing of their observations. The earth's motion, and that of 

 the few stars of which the parallax has been ascertained, would make 

 it probable that any alteration in position would not exceed a frac- 

 tion of the change which would have been observed by them. 



The author has since, for several years, devoted much time and 

 labour to this investigation, and believes that he has obtained a satis- 

 factory result. 



He refers to Doppler, who first suggested that the relative motion 

 of the luminous object and the observer would cause an alteration of 

 the wave-length of the light ; and to Ballot, Klinkerfues, Sonnche, 

 Fizeau, and Secchi, who have written on the subject. 



The author is permitted to enrich his paper with a statement of 

 the influence of the motions of the heavenly bodies on light, and of 

 some experiments made in an analogous direction, which he received 

 in June 1867 from Mr. J. C. Maxwell, F.R.S. 



It is shown that if the light of the star is due to the luminous 

 vapour of sodium or any other element which gives rise to vibrations 

 of definite period, or if the light of the star is absorbed by sodium- 

 vapour, so as to be deficient in vibrations of a definite period, then 

 the light, when it reaches the earth, will have an altered period of 

 vibration, which is to the period of sodium as V + v is to V, when V 

 is the velocity of light and v is the velocity of approach of the star to 

 the earth. Equal velocities of separation or approach give equal 

 changes of wave-length. 



§ II. Description of Apparatus. 



A new spectroscope is described, consisting in part of compound 

 prisms, which gives a dispersive power equal to nearly seven prisms of 

 60° of dense flint glass. Various methods were employed for the 

 purpose of ensuring perfect accuracy of relative position in the in- 

 strument between the star spectrum and the terrestrial spectrum 

 to be compared with it ; a new form of apparatus, which appears 

 to be trustworthy in this respect, was contrived. Many of the ob- 



