III. 



ON THE ZODIACAL LIGHT. 



By professor STEPHEN ALEXANDER. 

 (Read 1874, June 6.) 



As to the region of the zodiacal light M. Laplace, speaking 

 of the atmosphere of the sun, says: "The atmosphere at the 

 equator cannot extend beyond the point where tlie centrifugal 

 force exactly balances gravitation ; for it is manifest that beyond 

 that limit the fluid must itself be dissipated. As respects the 

 sun, that limit is at a distance from his centre of the radius of 

 the orbit of a planet which would complete its revolution in a 

 time equal to that of the rotation of the sun. The atmosphere 

 of the sun, therefore, does not extend even to the orbit of Mer- 

 cury; and consequently it docs not produce the zodiacal light, 

 which seems to extend even beyond the earth's orbit. Moreover, 

 this atmosphere, whose polar axis must be at least two-thirds of 

 that of the equator, is very far from having the lenticular form 

 which observations give to the zodiacal light."* 



Next, as to the origin and the constitution of the material which 

 gives us the zodiacal light, Laplace says: "If among the zones 

 abandoned by the atmosphere of the sun, there should be molecules 

 too volatile either themselves to combine, or to unite with the 

 planets, they ought while continuing to circulate about the sun, 

 to present all the phenomena of the zodiacal light, without op- 

 posing a sensible resistance to the diverse bodies of the planetary 

 system, either because of the extreme raritj' of those volatile 

 molecules, or because their motion is very nearly the same with 

 that of the planets which they encounter. "f 



It will be observed that the first of the two quotations here 

 made, estimates as probable that the material from which the 

 zodiacal light proceeds, itself extends beyond the earth's orbit. 

 This is in fact intimated by the existence of what in German ac- 

 counts of observations has been designated as the " gegeyischein," 

 which is seen in the part of the heavens opposite to the sun ; the 

 existence of which phenomenon is established by numerous ob- 

 servations, such especially as are detailed in various numbers of 

 the "Astronomische Nachrichten." 



Both eastern and western appearances are reported as occur- 

 ring simultaneously, by the late Rev. George Jones, A.M.,. 

 Ciiaplain in the U. S. Navy ; these phenomena are fully reported 

 in vol. Ill of the Report of the U. S. Japan Expedition, and the 

 extension of the light to both sides of the heavens is confirmed by 



* Systfeme da Monde, Book IV, chap. X. f Ibid., Note VII. 



9 (19) 



