Page:Popular Science Monthly Volume 11.djvu/369

Rh matter revolving round the sun, either within or beyond the orbit of the earth.

He starts with the proposition that such a ring must be either—1. Within the earth's orbit; 2. Surrounding the earth; or, 3. Without the orbit of the earth.

It cannot be within the earth's orbit; if so, we could never have it at any great altitude at any period of the night, and we could never have the zodiacal light at midnight on both horizons simultaneously.

We omit the diagram constructed for the consideration of the theory which supposes this ring to be around the sun and beyond the orbit of the earth, by which he shows that this supposition will not account for the lateral changes observed in the zodiacal light. He says:

"The query arises, 'Can such lateral changes, so uniformly observed, as the evening or morning advances, agree at all with the idea of a nebulous ring giving us this light at a distance from the spectator of 160,000,000 or 180,000,000 miles?' A ring of the character supposed, it seems to me, could give us a zodiacal light only of one uniform shape, namely, with the opposite borders receding from each other for a short distance from the apex, and then running parallel, one to the other, the whole way down to the base. Nor could the hourly changes of time produce any other changes in these boundaries than a rising or sinking of the apex of the light; the boundaries, say at nine o'clock, extending a little higher in the sky than at eight, and so continued, with a parallelism of the opposite sides, down to the horizon. How different this from the true facts of the case!

"The evident and most decided connection between these boundary-lines and the spectator's place, as regards the ecliptic, is also a matter of the greatest significance in drawing our conclusions in regard to the origin of this light. Now, supposing the base of the zodiacal light to be at a distance of 200,000,000 miles, how is it possible that the fact that the spectator is a short distance north or south of the ecliptic can govern the reflection from the nebulous ring at that immense distance, and place it on his side of the ecliptic? If he is on the north side of the ecliptic, not only is the main body of the light down to its base on that side, but the lateral changes of the boundaries, as the hours pass, are altogether or chiefly on that side; and so equally with the south. If he is on the ecliptic or near it, the zodiacal light stretches equally, or nearly so, on each side of that line. Also, if he changes rapidly during the night to or from the ecliptic (as was the case on shipboard) the boundaries of this light also change, being regulated by his motion (from one place to another). That the apex of the zodiacal light, from such a ring around the sun, might be so affected by the spectator's position is not unreasonable, but that the boundary-lines should be so affected seems to be utterly inadmissible.

"It is worthy of remark, also, how even and uniform, from apex to base, the change in the boundary-lines is as the hours change; as if the substance giving the zodiacal light were not only near, but also at one uniform distance from the spectator; the portions of it at the apex and base of the light all equally affected by his changes on the earth."

He shows by Bouguer's law of reflected light that the zodiacal light, if from a ring of nebulous matter beyond the earth's orbit,