SCINTILLATION OF THE STARS. 81 



and fruitless experiments with two alternately obscured lan- 

 terns. 



Horrebow and Du Hamel estimated the time occupied in 

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

 tance, according to Romer's first observations of Jupiter's satel- 

 lites, at 14' 7", then 11' ; Cassini at 14' 10" ; while Newton* 



* Newton, Optics, 2d ed. (London, 1718), p. 325. " Light moves 

 from the suu to us in seven or eight minutes of time." Newton com- 

 pares the velocity of sound (1140 feet in 1") with that of light. As, 

 from observations on the occupations of Jupiter's satellites (Newton's 

 death occurred 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 ve- 

 locity of light equal to 155,555| 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 calculations in the Jahrbuch fur 1852, an equatorial degree is 

 equal to 69-1637 English miles. According to Newton's data, we should 

 therefore have a velocity of 134,944 geographical miles. Newton, how- 

 ever, 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) 

 188,928 geographical, or 217,783 ordinary miles, in a second of time ; 

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

 markable, although the circumstance has been overlooked by Delambre 

 (Hist, de V Astronomie Moderne, torn, ii., p. 653), that Newton (proba- 

 bly basing his calculations upon more recent English observations of 

 the first satellite) should have approximated within 47" to the true re- 

 sult (namely, that of Struve, which is now generally adopted), while 

 the time assigned for the passage of light over the semi-diameter of the 

 earth's orbit continued to vacillate between the very high amounts of 

 11' and 14' 10", from the period of RBmer's discovery in 1675 to the be- 

 ginning of the eighteenth century. The first treatise in which R6mer, 

 the pupil of Heard, communicated his discovery to the Academy, bears 

 the date of November 22, 1675. He found, from observations of forty 

 emersions and immersions of Jupiter's satellites, " a retardation of light 

 amounting to 22 minutes for an interval of space double that of the sun's 

 distance from the earth." (Memoirs de V Acad, de 1666-1699, torn, x., 

 1730, p. 400.) Cassini does not deny the retardation, but he does not 

 concur in the amount of time given, because, as he erroneously argues, 

 different satellites presented different results. Du Hamel, secretary to 

 the Paris Academy (Regies Scienlianim Academics Historia, 1698, p. 

 143), gave from 10 to 11 minutes, seventeen years after Romer had left 

 Paris, although he refers to him ; yet we know, through Peter Horre- 

 bow (Basis Astronomic sive Triduum Roemerianum, 1735, p. 122-129), 

 that Romer adhered to the result of 11', when in 1704, six years before 

 his death, he purposed bringing out a work on the velocity of light; 

 the same was the case with Huygens ( Tract, de Lumine, cap. i., p. 7) 

 Cassini's method was very different ; he found 7' 5" for the first satel- 

 lite, and 14' 12" for the second, having taken 14' 10" for the basis of 

 his tables for Jupiter pro peragrando diametri semissi. The error was 

 therefore on the increase. (Compare Horrebow, Triduum, p. 129 ; Cas- 

 sini, Hypotheses et Satellites de Jupiter in the M6m de I' Acad., 1666- 



7) 2 



