Prof. Challis on a Theory of the Zodiacal Light. 183 

 so that the equation in (X, p,, v) is 



{far 1 — fr)\+ (rp'—r'p)/jL+ (pq'—p'q^} 



{pp'iqr'—qtrffiv + qq f (rp 1 —r ! p)v\ -\- rr'lpq' —p'q)Xp\ = ; 

 where the point represented by the equation 



(qr J —q t r)\ + (rp t —7 J p)p + (pq f —p , q)v=0 

 is, of course, the intersection of the two lines. 



XXIV. Supplement to a Theory of the Zodiacal Light. 

 By Professor Challis, M.A., F.R.S., F.R.A.S.* 



AT the end of my communication on the Zodiacal Light in 

 the February Number, reference is made to another pheno- 

 menon of like character, respecting which I expressed the opinion 

 that it is probably related to the zodiacal light, but I did not 

 attempt to give a theoretical explanation of it. Subsequent con- 

 sideration of the observations to which I there referred has led me 

 to the conclusion that it is so closely connected with the ordinarily 

 observed zodiacal light, that the same theory ought to embrace 

 both phenomena. The theory which I proposed should ac- 

 cordingly be capable of this extension ; and it is my present 

 object to show in what manner this demand upon it may be 

 satisfied. But first I must state the precise nature of the phe- 

 nomenon proposed to be explained, so far as that may be 

 gathered from the accounts of Mr. Jones and Prof. Brorsen, 

 which are the only ones that I am acquainted with. 



The description of the luminous appearance by Mr. Jones, 

 whose observations of the zodiacal light I referred to in my 

 former communication, is contained in a letter to the editor of 

 the ' Astronomical Journal ' (vol. v. p. 28), dated Quito, No- 

 vember 18, 1856. After describing the advantages of his posi- 

 tion in regard both to situation on the earth's surface relative to 

 the ecliptic, and the rarity and transparency of the atmosphere, 

 he makes a statement of the phenomena which had better be 

 given in his own words. " My observations," he says, " are 

 made on the summit of the hill Ychinbia, immediately adjoining 

 Quito on the east, a place where I have horizons sufficiently open 

 and clear. It is not my present object to give details of obser- 

 vations, but simply to notice one result, in hope of now drawing 

 attention to the subject, and perhaps of inducing other observers 

 to give it their particular notice. / see here every night, and all 

 through the night, a luminous arch from east to west, quite across 

 the sky. This arch, 20° wide, is visible at all hours, when the 

 sky is clear, but is brightest and most striking when the ecliptic 

 * Communicated by the Author. 



