Page:Popular Science Monthly Volume 11.djvu/370

would emit a light of the same intensity the whole way from the base to the apex. But another law of optics, that the strength of light is inversely as the squares of the distances of the object affording the light, would here make its application; and this ring, at our zenith, being by that supposition about 140,000,000 miles nearer to us, than at the base, we should then have the zodiacal light far more intense at the apex than at the base; all of which is entirely opposite to the facts of the case.

He offers, now, as a last conclusion, the hypothesis of a nebulous ring with the earth for a centre. He makes certain deductions from the examination of the preceding theories.

"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 character 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 sensibly affected by my own position on the 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 are untenable, is now the only one remaining to us.

"The hypothesis is that the zodiacal light is 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 our sister planet (Saturn), and also with the whole of Laplace's celebrated theory of the formation of globes."

We will not take up time and space with quoting his application of that theory in explaining some of the phenomena of this light.

Avoiding the consideration of these topics, we will proceed to apply the result of Bouguer's experiments on reflected light to this case.

In the annexed diagram he takes an observation made on the 4th of September, 1854, as an example, for the reason that it is a simple one, and one also in which the spectator is near the plane of the ecliptic. It was made in latitude 22° 18' north, longitude 114° 10' east. The sun rose at 5^h 48^m. The stronger light was at 3^h 30^m to 4^h 30^m, the diffuse light at 3^h 45^m. Sun's longitude 161° 35'. The horizons at 4^h 30^m, 3^h 30^m, 2^h 30^m, and 1^h 30^m, and at midnight, are given, together with the line of the spectator's vertices, as well as his positions O, o, etc., at 4^h 30^m and 3^h 30^m, A, B, C, F, are the boundaries of the zodiacal light at 4^h 30^m, and E, F, G, at 3^h 30^m; the apices C and G are carried a little above the more condensed portions of the ring; but the reader is at liberty to suppose them to be at any other part, as he may think best. The direction of the sun is given; and S', S", S'", S"", S'"", are supposed to be rays of light proceeding from that luminary.