SCINTILLATION OF THE STARS. 107 



of terrestrial measurement, lately conducted with much in- 

 genuity and success by M. Fizeau in the neighbourhood of 

 Paris. It reminds us of Galileo's early and fruitless experi- 

 ments with two alternately obscured lanterns. 



Horrebow and Du Hamel estimated the time occupied in 

 the passage of light from the sun to the earth at its mean 

 distance, according to Homer's first observations of Jupi- 

 ter's satellites, at 14' 7", Cassini, at 14' 10"; while Newton 48 



dent aux deux etats extremes, quand Algol, soit en diminuant, 

 soit en augmentant d* eclat, passe pour latroisieme grandeur/' 



" The attentive observation of the phases of Algol at a six- 

 month interval will serve to determine directly the velocity of 

 that star's light. Near the maximum and the minimum the 

 change of intensity is very slow ; it is, on the contrary, rapid 

 at certain intermediate epochs between those corresponding 

 to the two extremes, when Algol, either diminishing or in- 

 creasing in brightness, appears of the third magnitude. 



48 Newton, Opticks, 2nd ed. (London, 1718), p. 325. 

 " Light moves from the sun to us in seven or eight minutes 

 of time." Newton compares the velocity of sound (1140 

 feet in 1") with that of light. As, from observations on 

 the occultations of Jupiter's satellites (Newton's death oc- 

 curred about half a year before Bradley 's discovery of aberra- 

 tion) he calculates that light passes from the sun to the earth, a 

 distance, as he assumed, of 70 millions of miles, in 7' 30" ; this 

 result yields a velocity of light equal to 155555^ miles in a 

 second. The reduction of these [ordinary] to geographical 

 miles (60 to 1) is subject to variations according as we assume 

 the figure of the earth. According to Encke's accurate calcula- 

 tions in tiie Jahrbuch fur 1852, an equatorial degree is equal 

 to 6S-1637 English miles. According to Newton's data we 

 should therefore have a velocity of 134944 geographical miles. 

 Newton however assumed the sun's parallax to be 12". If 

 this, according to Encke's calculation of the transit of Venus, 

 be 8"'57116, the distance is greater, and we obtain for the 

 velocity of light (at seven and a half minutes) 188928 geo- 

 graphical, or 217783 ordinary miles, in a second of time; 

 therefore too much, as before we had too little. It is certainly 

 very remarkable, although the circumstance has been ovei- 



