6 PROPAGATION OF LIGHT. 



observed times was due to the change of distance. This 

 difference is found to be 8 m 13 s ; and accordingly the velocity 

 of light is such, that it traverses 192,500 miles in a second of 

 time. 



(10) The velocity of light, combined with that of the 

 Earth in its orbit, was afterwards applied by Bradley to explain 

 the phenomenon of the aberration of the fixed stars. From the 

 theory of aberration so explained, it followed that the velocity 

 of the light of the fixed stars is to the velocity of the Earth in 

 its orbit, as radius to the sine of the maximum aberration. 

 This latter quantity the constant of aberration, as it is called 

 is now found to be 20 /Xt 36 ; and the Earth's velocity being 

 known, the velocity of the light of the fixed stars is deduced. 

 The value so obtained is 191,500 miles in a second, which dif- 

 fers from that inferred from the eclipses of Jupiter's satellites, 

 by only the 2Uo^ n P ar ^ ^ ^ ne whole. 



From this it follows, that the direct light of the fixed stars, 

 and the reflected light of the satellites, travel with the same 

 velocity. 



(11) The velocity of light, emanating from a terrestrial 

 source, has been recently measured by M. Fizeau, by direct 

 experiment. The first idea of this experiment was communi- 

 cated to M. Arago, by the Abbe Laborde, a few years before : 

 its principle will be understood from the following description. 



Let the light of a lamp be reflected nearly perpendicularly 

 by a mirror placed at a considerable distance. Let a toothed 

 wheel, the breadth of whose teeth is equal to that of the interval 

 between them, be interposed near the luminous source ; and let 

 the mirror be so adjusted that the light passing through one 

 of these intervals is reflected to that diametrically opposite. If 

 the eye be placed behind the latter interval, the wheel being 

 at rest, it will perceive the reflected ray, which has traversed 

 a space equal to double the distance of the mirror from the 



