XXVI INTRODUCTION. 



raises as respects most of its properties: yet it is evident to our senses that Bougucr's law, that 

 the quantity of rays reflected is greater, the greater the angle "between the incident and reflected 

 light, applies to vapory particles (as we see in the reflection from clouds), and also to the mole- 

 cules of our atmosphere; and we may, I think, presume that it applies also to nebulous matter, 

 which, though seemingly a dense substance, has still nearly the transparency of our atmo- 

 sphere.* 



Eeferring now to the diagram, we find that, at the base of the Zodiacal Light, at 4 h 30 m , the 

 angle hetween the incident ray S B, and the reflected ray B E, is 15 ; at the base at 3 h 30 m , it is 

 25 ; at 2 h 30 m , it is 36 ; and at 1" 30 ra , it is 45. Thus the angles, we perceive, go on increas- 

 ing from the horizon at 4 h 30 up to that at midnight ; and allowing that, below 40 between 

 the incident and reflected ray, there is no perceptible difference in the quantity of the reflected 

 light, we should from this law have the Zodiacal Light of the same intensity the whole way, 

 from the base at 4 h 30, up to its apex. But the other law of optics referred to namely, that 

 the strength of light is inversely as the squares of the distance of the object affording the light 

 would here make its application; and this ring at our zenith being about 140,000,000 of miles 

 nearer to us than at the base at 4 h 30 m , we should then have the Zodiacal Light far more intense 

 at the apex than at the base; at the base, at 2 h 30 m it would be much stronger than at 3 h 30 1 "; 

 and at 3 1 ' 30'" stronger than at 4 h 30 ; all which is entirely opposite to the facts of the case. 



IX. 



I offer now, as a last conclusion, the hypothesis of a nebulous ring, with the earth for its centre. 

 There are certain deductions which appeared to come up in the examination of the preceding 

 theories, which I will now bring together, and exhibit in a united form. They are: 1. That 

 the substance giving us the Zodiacal Light must be equally near to us in all its parts, inasmuch 

 as the lateral changes of the Light i. e. the changes of boundaries have a uniform charac- 

 ter, and mostly a parallelism in their whole extent from apex to base; 2. That no part of this 

 substance can be very remote from us, inasmuch as the outlines of the Light were clearly and 

 decidedly affected by my own position on our globe, and even by my change of position, in a 

 single night ; and 3. That the laws of reflected light require an arrangement, or a shape, of this 

 nebulous matter, which will give us, at the base of the Zodiacal Light, larger angles between 

 the lines of the incident and reflected light, than at other portions, and also a regular decrease 

 of such angles from the base to the apex of the light, as produced by such a shape. These 

 three requirements appear to be fully met by an hypothesis, which, if the theories examined in 

 the preceding are untenable, is now the only one remaining to us. 



The hypothesis is that of a ring around the earth. 



The thought is a somewhat startling one, yet startling only from its novelty ; for it is entirely 

 in accordance with what we know of one of our sister planets (Saturn), and also with the whole 

 of Laplace's celebrated theory of the formation of globes. 



That great writer, after stating his ideas of the central condensation from an immense body 

 of nebulous, rotating matter, and thus of the formation of our sun, and of the rings about him, 

 produced from the remainder of that nebulous matter, thus proceeds : 



"If all the particles of a ring of vapors continued to condense without separating, they 

 would at length constitute a solid or a liquid ring. But the regularity which this formation 



I could, in clear nights, with the naked eye, easily make out stars of the Cth, and I sometimes thought of the 7th mag- 

 nitude, through its densest parts. 



