SCINTILLATION OF TIIK STAUS. 83 



creased. The result of these important observations gave 

 S' 17"78 ; from which, with a constant of aberration of 

 20"'4451, and Encke's correction of the smi's parallax in the 

 vear 1835, too-ether with his determination of the earth's 

 radius, as given in his Astro?i077iischcs Jahrbach fur 1852, 

 we obtain 166,196 geographical miles for the velocity of 

 light in a second. The probable error in the velocity seems 

 scarcely to amount to eight geographical miles. Struve's 

 result for the time which light requires to pass from the sun 

 to the earth differs about yf^^th from Delambre's (8' 13"-2), 

 which has been adopted by Bessel in the Tab. Regiom., and 

 has hitherto been followed in the Berlin Astronomical Al- 

 manac. The discussion on this subject can not, however, 

 be regarded as wholly at rest. Great doubts still exist as 

 to the earlier adopted conjecture that the velocity of the 

 light of the polar star was smaller than that of its compan- 

 ion in the ratio of 133 to 134. 



M. Fizeau, a physicist, distinguished alike for his great 

 acquirements and for the delicacy of his experiments, has 

 submitted the velocity of light to a terrestrial measurement, 

 by means of an ingeniously constructed apparatus, in which 

 artificial light (resembling stellar light) generated from oxy- 

 gen and hydrogen is made to pass back, by means of a mir- 

 ror between Suresne and La Butte Montmartre, over a dis- 

 tance of 28,321 feet, to the same point from which it ema- 

 nated. A disk having 720 teeth, which made 12-6 rotations 

 in a second, alternately obscured the ray of light and allowed 

 it to be seen between the teeth on the margin. It was sup- 

 posed from the marking of a counter (compteur) that the 

 artificial light traversed 56,642 feet, or the distance to and 

 from the stations in xg^-oot^ P^^'^ of a second, whence we ol)- 

 tain a velocity of 191,460 miles in a second. =^ This result, 

 therefore, approximates most closely to Delambre's (Avhicli 

 was 189,173 miles), as obtained from Jupiter's satellites. 



Direct observations and ingenious reflections on the ab- 

 sence of all coloration during the alternation of light in the 

 variable stars — a subject to which I shall revert in the se- 



* Fizeau gives his result in leagues, reckoning 25 (and consequently 

 44.52 metres) to the equatorial degree. He estimates the velocity of 

 light at 70,000 such leagues, or about 210,000 miles in the second. On 

 the earlier experiments of Fizeau, see Comptes Rendus, torn, xxix., p. 92. 

 In Moigno, Repert. d'Optique Moderne, Part iii., p. 1162, we find this 

 velocity given at 70,843 leagues (of 25=1°), or about 212,529 miles, 

 which approximates most nearly to the result of Bradley, as given by 

 Buech. 



