156 TIIE APPARENT POSITION OF THE ZODIACAL LIGIIT. 



region of the asteroids cannot account for all the phenomena of the zodiacal 

 light; we must in any case suppose that a great part of the light comes from 

 particles much nearer to the Sun. 



The various resemblances between different groups of phenomena which have 

 been pointed out in the preceding pages may be entirely fortuitous, and should 

 not be regarded as a satisfactory basis for any theory of the zodiacal light. 

 But it is only by taking notice of such resemblances when they occur that we 

 can be guided in subsequent observations. This consideration appears to me suf- 

 ficiently important to justify the foregoing discussion. Perhaps the results above 

 described might be either invalidated, or decidedly confirmed, by additional exami- 

 nation of the published work of observers of the zodiacal light, and especially of 

 the material provided by the energy of Jones. But the labor to be undertaken 

 in the necessary reductions is considerable, and the corrections to be applied 



must remain uncertain. Accurate knowledge of the phenomena of the zodiacal 

 light can be expected only from photometric observation. To whatever extent 

 these difficulties may hinder the derivation of additional knowledge from the work 

 of Jones, his memory will always be honored for the unusual assiduity which he 

 displayed in observations of the zodiacal light, and for his discovery of the first 

 law tending to explain its apparently irregular phenomena. 



Cassini's opinion, that the axis of the zodiacal light lies in the plane of the 



Sun's equator, was partly founded upon observation. But the observations by 

 Cassini, quoted by Jones at the end of his work, are insufficient to establish any 

 such conclusion. It seems from them, however, that Cassini saw the light gener- 

 ally farther to the north in Leo and Virgo than in Pisces and Aries, even after 

 making some allowance for the effect of absorption. This effect, on the other 

 hand, would sufficiently explain the northward tendency of the light during the 

 month of April, which suggested to Cassini the hypothesis above mentioned. On 

 merely theoretical grounds it has some claim to consideration ; for it is possible 

 that the rotation. of the Sun indicates the fundamental plane of the Solar System 

 if we accept the ordinary nebular hypothesis of its formation, more correctly than 

 can be clone by the revolutions of the known planets. Since the longitude of 

 the ascending node of the Sun's equator is about 75°, the results of the present 

 inquiry agree tolerably well with Cassini's hypothesis. 



The reduction of fifty-eight observations, by various observers, of the vertex 



> 



of the zodiacal light, made by Houzeau in 1844, 1 gave for the longitude of 



1 Astronomische Nachrichten, XXI. 186. 



