INTRODUCTION. XIX 



V. 



May not tins Light be from a nebulous body of lenticular shape reaching to the sun, and lying 

 in, or near, the plane of the ecliptic ? 



This query, I believe, comprehends Mairan's theory; and also that of many others of our own 

 time, who, having abandoned part of his opinions, still hold to the lenticular shape. Mairan's 

 idea of such a body connected with the sun, and revolving with that body, must be abandoned, 

 for the reasons shown in page vii of this work. But may there, still, not be such a lenticular- 

 shaped body, with the sun for its centre, yet not revolving with that luminary, but having a 

 rotatory motion of its own? 



The idea of a lenticular shape has doubtless arisen from the peculiar outline of the Zodiacal 

 Light as presented, in its best and strongest aspects, to our eye. This Light being somewhat 

 lenticular in shape, the body producing it is supposed to be similar in shape. But we must 

 remember that, in order to draw such an inference, we must be without this lenticular body, 

 looking down upon it; and that, to admit of this, its extent must fall short of our earth. But if 

 it falls short of our earth, I could not have had the Zodiacal Light on both the eastern and west- 

 ern horizons at midnight; nor could it, in any observation, have reached far up in the sky; it 

 could never, by any possibility, reach the spectator's zenith, which the Zodiacal Light is known 

 very often to do. 



If we suppose this lenticular nebulous body to involve the earth, and the spectator to be in its 

 ruidst, looking through it, then the argument for a lenticular shape loses its force ; for any other 

 shape may give us such a result, if such a result can be given at all. But we may very well 

 query, whether, if involved in the nebulous matter, and looking through it, we could get any 

 distinct outlines by reflection from its particles, any more than, when involved in a fog, or in 

 smoke, or in our earth's atmosphere, we can get their outlines. It is true that, when extrane- 

 ous matters are mingled with these, we may get marked lines or boundaries in them, as we often 

 do in our atmosphere from humidity along the horizon ; but all this is from causes extraneous, 

 and is not from the atmosphere itself; and it is also temporary in its nature. 



It may also be said, that if we are involved in this nebulous matter, portions of it may give 

 a reflection reaching our eye, while others do not ; as it was asserted in I that this matter 

 actually does. Yet still it may be a subject of strong doubt, whether, if thus involved, the 

 reflections could come to us with any such strongly-marked character, and with such distinct 

 outlines, any more than they can come from our own atmosphere alone, while we are involved 

 in its particles. My own impression is, that they could not. 



There are, however, other difficulties in the way of this theory, which I will proceed to state: 



1. This lenticular body having, in order to sustain itself, to rotate on an axis (which axis 

 must be at the sun), and its diameter being more than 190,000,000 of miles, its portions near 

 the centre and at its outer edge would require velocities entirely incompatible with each other. 

 Either the inner portion would be drawn to the sun, from the want of centrifugal force; or the 

 outer would fly off and be lost, from an excess of it; or the whole body, more probably, would 

 soon resolve itself into a series of concentric rings, with intervals between them, somewhat accord- 

 ing to Laplace's theory of the formation of our planetary system. But that there are no such 

 concentric rings in this case, giving, by their combined effect, the Zodiacal Light, is evident 

 from the fact that the light is continuuus and regular, not broken into cross-bauds or mottled, 

 as would be the case, if from a number of concentric rings. 



