116 PROCEEDINGS OF THE AMERICAN ACADEMY 



The second process which Astronomy has supplied for obtaining the 

 velocity of light may be called the indirect method. It demands not a 

 space, but a velocity which is commensurable with the velocity of light. 

 If two such velocities are compounded together, according to the prin- 

 ciple of the parallelogram of motions, there is a resultant motion, the 

 direction of which deviates sensibly from that even of the largest mo- 

 tion which enters into the composition. In nature the velocity of the 

 earth is compounded, in this way, with the velocity of light, and imparts 

 an apparent path to the light, differing by a small angle from the true 

 path. The angular displacement which this causes between the appar- 

 ent and real places of a star is called aberration, and was first discov- 

 ered by Bradley, in 1726 ; this astronomer explaining, on this simple 

 principle, anomalies in observation which had hitherto been considered 

 accidental. As the displacement of the star works opposite ways at 

 opposite seasons of the year, half the difference between the extreme 

 places is the distance from the apparent to the true place, or the 

 constant of aberration. This, when known as an observed fact, estab- 

 lishes the ratio between the velocity of light and the velocity of the 

 earth, and enables the astronomer to assign the value of the one with 

 all the accuracy which pertains to his knowledge of the other. Ac- 

 cepting Struve's determination of the aberration, viz. 20".45, the 

 velocity of light is calculated to be 10088 times as great as the velocity 

 of the earth. The mean velocity of the earth is known with all the 

 certainty which belongs to our knowledge of the magnitude of the 

 earth's orbit ; that is, of the sun's distance. Assuming, as before, that 

 the distance derived from Encke's parallax is the most reliable, the 

 velocity of the earth in one second of solar time is 18.987-|- miles. This 

 multiplied by the aforesaid ratio gives 191513 miles for the velocity 

 of light by Bradley's method. It appears, therefore, that the velocities 

 by the two methods of Astronomy (the direct and the indirect), differ 

 by 1837 miles ; a small quantity comparatively, being only one per 

 cent of the whole velocity. Whatever other value is adopted for the 

 sun's distance will alter these two results proportionally, without dis- 

 turbing the ratio between them. I may add, that the velocity which 

 aberration ascribes to light belongs to it at the earth's surface ; that is, 

 in the dense atmosphere ; whereas, the velocity discovered from the 

 eclipses is that which extends through the planetary spaces. This 

 distinction, however, will do little towards bringing the two results into 

 greater accordance. The velocities of light in different media are pro- 



