Closing Years of the Nineteenth Century. 415 



other at the time when the light was emitted, resolved along 

 the line of sight. In the existing state of double-star astronomy, 

 this effect would be masked by errors of observation. 



Villarceau also examined the consequences of supposing 

 that the velocity of light depends on the velocity of the source 

 by which it is emitted. If, for instance, the velocity of light 

 from a star occulted by the moon were less than the velocity of 

 light reflected by the moon, then the apparent position of the 

 lunar disk would be more advanced in its movement than that 

 of the star, so that at emersion the star would first appear at 

 some distance outside the lunar disk, and at immersion the star 

 would be projected on the interior of the disk at the instant of 

 its disappearance. The amount by which the image of the star 

 could encroach on that of the disk on this account could not be 

 so much as 0"'71 ; encroachment to the extent of more than 

 1" has been observed, but is evidently to be attributed for the 

 most part to other causes. 



Among the consequences of the finite velocity of propagation 

 of light which are of importance in astronomy, a leading place 

 must be assigned to the principle enunciated in 1842 by Christian 

 Doppler,* that the motion of a source of light relative to an 

 observer modifies the period of the disturbance which is 

 received by him. The phenomenon resembles the depression 

 of the pitch of a note when the source of sound is receding from 

 the observer. In either case, the period of the vibrations 

 perceived by the observer is (c + v) / c x the natural period, 

 where v denotes the velocity of separation of the source and 

 observer, and c denotes the velocity of propagation of the 

 disturbance. If, e.g., the velocity of separation is equal to 

 the orbital velocity of the earth, the D lines of sodium in the 

 spectrum of the source will be displaced towards the red, as 

 compared with lines derived from a terrestrial sodium flame, bs 

 about one-tenth of the distance between them. The application 

 of this principle to the determination of the relative velocity of 



* Abhandl. der K. Hohm. Ges. der Wissensch. (5) ii (1842), p. 465. 



