SCINTILLATION OF THE STARS. Si 



and fruitless experiments with two alternately obscured Ian 

 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 observationsof Jupiter's satel- 

 lites, at I'V 7", then 11' ; Gassini 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 I") witli that of light. As, 

 from observations ou the occultatious of Jupiter's satellites (Newton'a 

 death occurred about half a j^ear before Bradley's discovery of aberra- 

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

 as he assumed, of 70 millions of miles, iu 7' 30" ; this result yields a ve- 

 Jocity of light equal to 155,555 1 miles in a second. The reduction of 

 these [ordinary] to geographical miles (60 to 1°) is subject to variations 

 according as we assume tlie figure of the earth. According to Encke's 

 accurate calculations in the Jahrbtich fur 1852, an equatorial degree is 

 equal to 60-1637 English miles. According to Nesvton'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 ordiuaiy 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 

 tiie time assigned for the passa2:e of li^ht over the serni-diameter of the 

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

 11' and 14' 10'', from the period of Romer's discovery iu 1675 to the be- 

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

 the pupil of Picard, 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, tom. 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, 

 ditferent satellites presented different results. Du Hamel, secretary to 

 the Paris Academy {Regies Scientiamm Acaderuice Historia, 1698, p. 

 143), gave fi-om 10 to 11 minutes, seventeen years after Romer had left 

 Paris, althoush he refers to him; vet we know, through Peter Horre- 

 bow {Basis Asfronomice sive Triduum Roemeriannm, 1735, p. 122-129), 

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

 his death, he purposed bringing out a w^ork on the velocity of light; 

 the same was the case with Huy^ens {Tract, de Lnmine, 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 .Tupiter pro pcragrando diametri semissi. The error wa« 

 therefore on the increase. (Compare Hon-ebow, Tridnnm, p. 129 ; Cas- 

 Hini, Hi/potheses et SatcUUes de Jnj^iter in the M^ni. de VArnd., J 666- 



T) 2 



