﻿Mr. It. A, Proctor on the Zodiacal Light. 61 



to a great circle of the heavens, and that the whole of the visible 

 heavens south or north of that great circle would be hidden by 

 the luminous disk. In other words, a region of the heavens far 

 larger than that occupied by the arch of Liais, or by the eastern 

 and western lights of Jones, should be occupied by the zodiacal 

 light if it had some such extension as we have assumed in the 

 case of this luminous disk. 



It is to be remembered, however, that, assuming (as we are 

 bound to do) a considerable degree of flatness in the actual figure 

 of the zodiacal disk, and more especially of its more distant por- 

 tions, then much more light would be received from those parts 

 towards which the line of sight is directed at a considerably acute 

 angle, than from those parts which the line of sight crosses nearly 

 at right angles. And it is easy to see that, on any reasonable 

 assumption as to the range of zodiacal substance which it is ne- 

 cessary that the line of sight should traverse in order that any 

 appreciable light may be received, the occasional visibility of 

 the light where the superior planets alone can be seen becomes 

 as readily explicable as the ordinary visibility of the light in those 

 parts of the sky where the inferior planets become visible. 



It will be seen that all that can be strictly said to have been 

 demonstrated in this paper is the fact that the zodiacal light is 

 associated with the sun, and not with the earth, — that it is not 

 due to solar light reflected from bodies travelling within the 

 earth's orbit, whether in circular or elliptic orbits, — and that, if 

 the major part of the zodiacal light is reflected solar light, then 

 the paths of the bodies reflecting that light must resemble those 

 of the meteors encountered by the earth. As the spectroscope 

 seems to show that at least a portion of the light* of the zodiacal 

 gleam is not reflected solar light, we cannot, in the present state 

 of our knowledge, definitely decide on a theory as to the motions 

 of the bodies to which the light is due ; for the solution of the 

 problem is obviously bound up with the interpretation of the 

 physical nature of the zodiacal light. If some solar action, for 

 example, rouses luminosity in certain definite directions (as, for 

 instance, near the plane of the sun's equator) in some such way 

 as light is caused to appear along radial lines through and be- 

 yond the heads of comets, our power of theorizing from such 

 considerations as have been dealt with in this paper would be 



* I use this mode of speaking, not by any means as doubting tbe accuracy 



o 



of Angstrom's observation, but because, even if the greater part of the light 

 gave a continuous spectrum, yet this spectrum might remain undiscernible 

 even when bright lines corresponding to a very minute proportion of the 



total light were seen with ease. Nay, such bright lines as Angstrom found 

 in the spectrum of the phosphorescent light from the sky might be detected 

 when a continuous spectrum from the much brighter light of the zodiacal 

 radiance remained unseen. 



