﻿60 Mr. R. A. Proctor on the Zodiacal Light. 



would (at least as respects distribution along their several paths 

 and the general figure of those paths) resemble very closely the 

 meteoric systems which, as we know, the earth traverses in the 

 course of her annual motion around the sun. 



By considering the zodiacal light we have thus been led to a 

 theory involving, and associated with, the theory of meteor- 

 systems as now established by the labours of Adams, Leverrier, 

 Schiaparelli, and others. But it is worth noticing that, by rever- 

 sing the process, and considering first the theory of meteor- 

 systems so established, we are led quite as readily to the theory 

 that there must at all times exist in the sun's neighbourhood a 

 disk of discrete constituents which would present precisely such 

 an appearance as the zodiacal light. I have shown elsewhere 

 that this result is a simple mathematical deduction from the 

 evidence. 



But setting this consideration wholly on one side, the fact re- 

 mains that all other theories of the zodiacal light (that is, of the 

 motions of its constituent parts, without reference to its physical 

 constitution) have been eliminated. It remains only to be shown 

 that this theory is controverted by no peculiarities in the ob- 

 served appearance of the zodiacal light, and also that we should 

 inquire what farther general laws, if any, may be predicated of 

 the motions of the bodies composing this object. 



The fact that the axis of the zodiacal light is ordinarily close 

 to the ecliptic, is accounted for on the assumption that the various 

 paths along which the constituents of the zodiacal disk travel, 

 tend to aggregate towards the neighbourhood of the ecliptic. 

 There is nothing, however, to prevent individual systems from 

 having a considerable inclination to that plane. 



The observed variation of the zodiacal light in brilliancy, posi- 

 tion, and extent is obviously to be expected according to the view 

 of its structure now under consideration. 



The simultaneous appearance of an eastern and western light 

 and Liaises observation of a complete arch of light have to be 

 accounted for as highly exceptional but, at the same time, recog- 

 nized phenomena. It is easy to see that both these phenomena 

 may be regarded as indicating the occasional but very exceptional 

 extension of the zodiacal disk to a considerable distance beyond 

 the orbit of the earth. But it must not be concealed that there 

 are great difficulties to be removed before this interpretation can 

 be regarded as satisfactory. 



Let us suppose, for instance, the case of a thin luminous disk 

 occupying the whole orbit of Mars, and that the earth is in the 

 part of her orbit where her distance north or south of this plane 

 is greatest. Then it will be evident that the outline of the disk, 

 as seen from any part of the earth, would correspond very nearly 



