﻿Mr. R. A. Proctor on the Zodiacal Light. 59 



has varied at times in the most remarkable manner, while its 

 luminosity has been so variable that sometimes for months to- 

 gether it has been scarcely perceptible (in our northern lati- 

 tudes) ; while at others it has been singularly conspicuous. I 

 set on one side for the moment those observations by Lieutenant 

 Jones which would imply that at times the zodiacal light in- 

 creases so greatly in extent as to become visible at once both on 

 the eastern and western horizon. I also set on one side those 

 observations by M. Liais, according to which the zodiacal light 

 can be seen at times extending as a complete arch from the 

 eastern to the western horizon. Assuming these observations to 

 be reliable (and those by M. Liais do not seem open to ques- 

 tion), a true theory of the zodiacal light may be expected to 

 account for them. But without insisting on this, it is evident, 

 I think, that the admitted variations of the zodiacal light, in 

 position, extent, and splendour, do not admit of being interpreted 

 by the theory that the light is due to a disk including always 

 the same materials moving in orbits of small eccentricity. 



Nor do our difficulties seem removed if we assume that the 

 constituents of the disk travel in orbits of considerable eccen- 

 tricity, so long as we suppose that the actual constitution of 

 the disk is constant, or nearly so, amidst whatever variations in 

 the distribution of individual constituents. 



Yet the general aspect of the zodiacal light, and the consider- 

 ations already applied to other theories, suffice to prove that 

 there is always present around the sun as centre a disk, whether 

 composed of discrete meteorites, of vaporous masses, or of some 

 combination of these and other forms of matter. The materials 

 of this disk must be in motion around the sun in accordance 

 with the laws of gravity ; at least we have no evidence whatever 

 inviting us to the supposition that they differ in this respect 

 from all the other constituents of the solar system. 



We are thus led to the conclusion that the bodies composing 

 the zodiacal light travel in orbits of considerable eccentricity, 

 carrying them far beyond the limits of what we may now term 

 the zodiacal disk. The constitution of the disk thus becomes 

 variable, and that within limits which may be exceedingly wide. 

 They must be so, in fact, if all the recorded variations of the 

 zodiacal light are to be accounted for. In other words, it is 

 requisite (if our evidence is to be explained) not only that the 

 paths of the materials composing the zodiacal light shall be for 

 the most part very eccentric, but that along those paths the 

 materials should not be strewn in such a way that a given por- 

 tion of any path is at all times occupied by a constant, or nearly 

 constant quantity of matter. 



According to this view, the constituents of the zodiacal light 



