XX INTRODUCTION. 



2. In tlie annexed figure, constructed to elucidate this theory, let A B C D Gr represent the 

 outer edge of this supposed nebulous lenticular body ; S, the sun ; E, the earth ; let the line Gr 

 H be in the plane of the horizon of a spectator on the earth; taking for illustration the obser- 

 vation of September 4th, 1854, at 4 h 30 m a. m.; which observation is selected, not because it is 

 of more applicability than any others in the book, but because it is a simple case, and because 

 the observer at 4 h 30 m being near the ecliptic, a cross section a b c c d d of the lenticular body 

 would be very nearly in the plane of his horizon; so nearly, that, in our reasoning, they may be 

 considered the same. The lines of the spectator's horizons, of sufficient correctness for illustra- 

 tion, back to midnight, and also lines to his zeniths, are given on the diagram. The lines eee, 

 fff, g g y, li h h, i i i, may represent the boundaries of the Zodiacal Light at 4 h 30 m , 3 b 30 m , 



2 h 30 m , l b 30 m , and midnight ; the points where each set strikes the horizons or sections abed, 

 &c v in the nebulous lenticular body, being at the option of the reader. 



Now it will be obvious, that, from whatever portions of the lenticular body the Zodiacal Light 

 comes reflected to the spectator at E, it must grow narrower as we recede back from the hori- 

 zon at 4 b 30 m , by the different horizons to that of midnight, the base g g, in the horizon at 2 h 

 30 m , being much narrower than that of e e, in the horizon of 4 h 30 m ; and so to midnight, when 

 the width must become very greatly reduced. But there is no such change in the actual width 

 of the Zodiacal Light itself; but, on the contrary, though the various hours of the night make 

 themselves felt decidedly in its brightness, they give us no perceptible change in its breadth; 

 the base of the great body of the Light at 2 h 30 m having, usually, as great a width as at the 

 dawn. Certainly there is no such a regular increase of breadth in the morning, or reduction in 

 the evening, as such a theory would necessarily demand. 



3. Objection 1 could be obviated by supposing the inner portion of this lenticular body re- 

 moved, and only the outer part ABCDLMNO left as a ring enveloping the earth, retaining 

 still its lenticular shape, of which A L M would represent a cross-section. In that case the lines 

 a b e would have to represent the boundaries of the Zodiacal Light ; and it will be seen that 

 from, somewhere about Oto a b, these lines would have to be nearly parallel to each other, while 

 from to e they would have a different course; and, indeed, the whole shape of the Zodiacal 

 Light would change, and be different in the different hours of the night, according as the hori- 

 zons change; whereas one of the greatest characteristics of this Light is the regular uniformity 

 of its shape. 



4. There are two great laws with regard to light, from which a strong objection to this theory 

 would probably arise ; but it is difficult to apply them, in consequence of the uncertainty re- 

 specting those portions of the lenticular body from which the Zodiacal Light may be supposed 

 to proceed. The laws referred to are : 1, That when light is reflected to the eye, the quantity 

 reflected is in proportion to the greatness of the angle between the lines of incident and reflected 

 light; and 2, that the strength of light is inversely as the squares of the distance of the object 

 from which it proceeds. I shall, when considering Laplace's theory, apply these laws with 

 more particularity; but, on account of the uncertainty referred to, can only advert to them in 

 the present case. As the reader may wish to return to this diagram, and to study their appli- 

 cation to it, I will add here the dimensions of the angles, which, in the imaginary case, 

 would lie between the lines of incidence and reflection. They are, S m E, 91; S H E, 97; 

 S o E, 89 ; S r E, 89 ; S t E, 74 ; S v E, 21 30' ; S w E, 38. 



5. It appears to me, also, that if this substance involved the earth, the earth and moon, or 

 one of them at least, would feel its effects upon their motions. The moon, in its revolution 



