﻿58 Mr. E-. A. Proctor on the Zodiacal Light. 



the earth, whether that ring be close by the earth or at a dis- 

 tance comparable with the moon's*. 



- We need not consider the theory that the light may be due 

 to a self-luminous ring around the earth, for obvious reasons. 



Now, passing from the normal features of the ring to more or 

 less exceptional peculiarities, we find ourselves compelled to re- 

 ject yet one other theory of the light : I mean the theory that it 

 is due to a disk of minute bodies travelling in orbits of small 

 eccentricity around the sun. 



The peculiarities which oppose themselves most strikingly to 

 this theory are those which relate to the position and extent of 

 the zodiacal light, though it will be obvious that the observed 

 variations in the apparent brightness of the light are not readily 

 explicable on this hypothesis. 



Admitting the existence of a disk of bodies travelling as sup- 

 posed, it will be evident that the changes affecting the motions 

 of any member of the system would correspond exactly to those 

 which would affect the motions of any considerable orb travel- 

 ling at a similar distance from the sun. In other words, the 

 changes would resemble those slow periodic changes which 

 affect the orbits of the earth, Venus, and Mercury. Nor is it 

 conceivable that the members of the system would so interfere 

 with each other's motions as to affect appreciably at any time the 

 appearance of the disk. Now changes such as these, affecting 

 the individuals of a set of bodies which at any one time were 

 spread with a certain uniformity (as the ordinary appearance of 

 the zodiacal light would imply to be the case with its constitu- 

 ents), could not account for the observed changes in the position 

 and extent of the light. The axis of the gleam has been seen 

 at times, by practised observers, inclined at a considerable angle 

 to the plane of the ecliptic. The extent of the zodiacal light 



* While dealing with the relations presented by the Saturnian ring- 

 system, in 1864, 1 was led to apply the formula, with suitable changes of 

 elements, to the ease of a ring circling the earth, being invited to the in- 

 quiry by the perusal of the observations made by Lieutenant Jones, and 

 comments made thereon by Baron Humboldt. I found that there is not a 

 single hypothesis as to the dimensions of such a ring which would lead to 

 results according with, or even in the slightest degree approaching, the 

 results of observations made upon, the zodiacal light. This conclusion is 

 embodied in a note at p. 117 of that treatise ; in which note I remark that 

 such investigations "prove that the zodiacal light cannot be due to a ring 

 of a minute satellite surrounding the earth, the appearance of the ring in 

 high latitudes being altogether different from that which would be pre- 

 sented by a ring surrounding the earth. lam careful to refer to these re- 

 searches and their results, because remarks have been published implying 

 that I have somewhat hastily come to a decision on the points here dealt 

 with. A complete mathematical investigation of the subject made fully 

 six years since may be regarded as fairly meeting those remarks. 



