110 COSMOS. 



of these numbers should be considerably increased. The 

 result of these important observations gave 8' 17"' 78; from 

 which, with a constant of aberration of 20"-4451, andEncke's 

 correction of the sun's parallax in the year 1835, together 

 with his determination of the earth's radius, as given in his 

 Astronomisches Jahrbuch fur 1852, we obtain 166196 geo- 

 graphical 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 

 T i^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 Almanack. The discussion on this 

 subject cannot, 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 companion 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 sub- 

 mitted the velocity of light to a terrestrial measurement, by 

 means of an ingeniously constructed apparatus, in which arti- 

 ficial light (resembling stellar light) generated from oxygen and 

 hydrogen, is made to pass back by means of a mirror between 

 Suresne and La Butte Montmartre, over a distance of 28321 feet 

 to the same point from which it emanated. A disc having 720 

 teeth, which made 12*6 rotations in a second, alternately ob- 



to 215834 miles, and approximates most nearly to Struve's 

 recent result, while that obtained at the Pulkowa Obser- 

 vatory is 189746 miles. On the difference in the aberra- 

 tion of the light of the Polar star and that of its companion, 

 and on the doubts recently expressed by Struve, see Madler, 

 Astronomic, 1849, s. 393. William Richardson gives as the 

 result of the passage of light from the sun to the earth 8' 1 9"'28, 

 from which we obtain a velocity of 215392 miles in a second. 

 (Mem. of the Astron. Soc., vol. iv. P. i. p. 68.) 



