FOOTNOTES:
[23] Professor Kirkwood has published a most interesting series of inquiries, going far to prove that the real secret or the planetary influences lies in the fact that the sun’s surface is not uniform, and that on a certain solar longitude the planetary influences are more effective than elsewhere.
[24] To these may be added the following law:
4. Light reflected from any opaque body gives the same spectrum as it would have given before reflection.
5. But if the opaque body be surrounded by vapors, the dark lines corresponding to these vapors make their appearance in the spectrum with a distinctness proportioned to the extent to which the light has penetrated those vapors before being reflected to us.
6. If the reflecting body be itself luminous, the spectrum belonging to it is superadded to the spectrum belonging to the reflected light.
7. Glowing vapors surrounding an incandescent source of light may cause bright lines or dark lines to appear in the spectrum, according as they are more or less heated; or, they may emit just so much light as to make up for what they absorb, in which case there will remain no trace of their presence.
8. The electric spark presents a bright-line spectrum, compounded of the spectra belonging to the vapors of those substances between which, and of those through which, the discharge takes place. According to the nature of these vapors and of the discharge itself, the relative intensity of the component parts of the spectrum will be variable.
Lastly, the appearance of the spectrum belonging to any element will vary according to the circumstances of pressure and temperature under which the element may emit light.
[25] It is also shown most conclusively, by a photograph of the eclipse of August, 1868, taken an instant before the totality. Here we see the glare trenching upon the moon’s disk (elsewhere black), as it should theoretically. So soon as totality commenced, the glare had reached the moon’s limb, whence it must immediately have passed quickly away.
[26] In fact, if we take the mode of reasoning by which Mr. Lockyer has endeavored to get over certain physical difficulties presently to be mentioned, we shall be able to point definitely to the place where his argument fails. He says, conceive a tiny moon placed so as to appear coincident with the centre of the sun’s disk. There will be atmospheric glare as well as direct sunlight. Now, conceive this small moon to expand until it all but covers the sun. Still there will be glare and a certain small proportion of direct sunlight. So far his reasoning is most just. But when he allows his expanding moon to cover the sun, and to extend beyond the solar disk as in total eclipse, the atmospheric glare can no longer be assumed to exist all round the expanding moon: at the moment when the moon just hides the sun, the glare begins to leave the moon, a gradually expanding black ring being formed round that body. It is only necessary to consider where the glare comes from to see that this must be so.
I have taken no account of diffraction here, because it has been abundantly proved that no corona of appreciable width could be formed around the moon during total eclipse by the diffraction of the rays of light as they pass near the moon’s limb.