258 VeloHty of Light 



Almanac. The discussion of this subject cannot be regarded as 

 connpletely terminated ; but the earlier entertained supposition, that 

 the velocity of the light of the Pole-star was less than that of its 

 companion in the ratio of 133 : 134, remains subject to great doubts. 



A physicist distinguished for his knowledge as well as for his great 

 delicacy in experimenting, M. Fizeau, has succeeded in executing a 

 terrestrial measurement of the velocity of light, by means of an inge- 

 niously devised apparatus, in which the artificial star-like light of 

 oxygen and hydrogen is returned to the point from whence it came, 

 by a mirror placed at a distance of 8633 metres (28,324 English 

 feet), between Suresne and La Butte Montmartre. A disc, fur- 

 nished with 720 teeth, which made 12-6 revolutions in a second, 

 alternately stopped the ray of light, and allowed it to pass freely be- 

 tween the teeth of the limb. From the indications of a counter 

 (compteur) it was inferred, that the artificial light traversed 17,266 

 metres (56,648 English feet), or twice the distance between the sta- 

 tions, in x¥oOo ^^ ^ second of time ; whence there results a velocity 

 of 167,528 geographical miles in a second. This result comes nearest 

 to that of Delambre derived from Jupiter's satellites, which is 167,976 

 geographical miles in a second. 



Direct observations, and ingenious considerations on the absence of 

 any alteration of colour during the change of light of variable stars, 

 (a subject to which I shall presently return), have led Arago to the 

 conclusion that (in the language of the undulatory theory), rays of 

 light which 'have different colours, and therefore very different 

 lengths and rapidities of transverse vibration, move through space 

 with equal velocities ; but that in the interior of the different bodies 

 through which the coloured rays pass, their rates of propagation and 

 their refractions are different. Arago''s observations have shewn, 

 that in the prism the refraction is not altered by the relation which 

 the velocity of light bears to that of the earth's motion. All the 

 measurements accord in the result, that the light of the stars towards 

 which the earth is advancing, has the same index of refraction, as 

 the light of the stars from which the earth is receding. Speaking in 

 the language of the emission hypothesis, the celebrated observer we 

 have just named said, that bodies send forth rays of all velocities, 

 but that among these different velocities there is only one which can 

 awaken the sensation of light. 



If we compare the velocities of solar, sidereal, and terrestrial light, 

 which all comport themselves exactly in the same manner in the 

 prism, with the velocity of the current of friction-electricity, we are 

 inclined to assign to the latter, according to the experiments devised 

 with admirable ingenuity by Wheatstone, a velocity superior to the 

 former in the ratio of at least 3 to 2. According to the lowest results 

 of Wheatstone's optical rotating apparatus, the electric current tra- 

 verses 288,000 English statute miles, or 250,000 geographical miles, 

 in a second. If. then, we reckon with Struve for sidereal light in the 



