xlviii Appendix. 



cannot do better than quote what he says on this subject [Extract read ; 

 Keith Johnston's " Atlas of Astronomy," page 5.] I have brought with me 

 the illustration referred to by Mr. Hind which, judging from my own observa- 

 tion and from the accounts of others, conveys a fair representation of the 

 phenomenon. 



With regard to the time of the year during which the Zodiacal Light is 

 visible in England, the record of the Rev. T. W. Webb in " Nature," 8th 

 February, 1872 (vol. v., p. 285), of the latest observations there, of which I 

 have seen an acount, corroborates what Mr. Hind says on the subject. 



Of the only observation of the Zodiacal Light which I have had the 

 opportunity of making in this hemisphere, I can only spetik from memory. 

 It was during the winter months, and the apex of the cone of light, which, 

 was on that occasion defined with more than usual clearness, was near one of 

 the briohter stars in the constellation Leo. The sun was at the time far below 

 the horizon, and the distance of the apex from the horizon was fully 40°. 

 A reference to Mr. Skey's diagram will show that if the figure he has given 

 as an approximation to that of the Zodiacal Light is to be taken as an 

 essential detail of his hypothesis, this observation, and indeed all observations 

 during our winter months, decidedly invalidate it. As I have already said, 

 this particular detail does not appear to be essential to the theory, a,lthough it 

 renders it desirable that the manner in which it has been expressed should be 

 revised. The general form of the envelope from which we derive the Zodiacal 

 Light may be somewhat as Mr. Skey has supposed, but it is necessary to 

 admit of a very considerable extension in all directions from the sun in and 

 near the plane of his equator, in order to account for its visibility throughout 

 the year. 



Various hypotheses as to the constitution of this solar envelope have been 

 put forward. Sir John Herschel speaks of it in his " Outlines of Astronomy," 

 paragraphs 897 and 898. Becquerel, in a recent work, gives the prevalent 

 opinion among French physicists as follows : — " Many explanations of this 

 phenomenon have been offered, the most probable being that which considers 

 it due to a group of bodies which form, as it were, a zone around the sun 

 of solid asteroids, widely separated from one another, but occupying an 

 enormous space, in the midst of which the earth is plunged ; aerolites and 

 shooting stars will then be but isolated bodies belonging to this group, 

 which, drawn within the sphere of the earth's activity, fall upon its surface. 

 According to this hypothesis, the Zodiacal Light will be due to reflected 

 solar light, and the absence of polarization which has been observed in it is 

 a result of the light being reflected in all possible planes from the variously 

 presented surfaces of this multitude of bodies." — (Becquerel, La Lumiere 

 ses Causes et ses Effets, Tome I., p. 7, 1867.) Mr. Skey, as you are aware. 



