[1] More strictly, it plays the same part as a glass screen before a glowing fire. When the heat of the fire falls on such a screen (through which light passes readily enough), it is received by the glass, warming the glass up to a certain point, and the warmed glass emits in all directions the heat so received; thus scattering over a large space the rays which, but for the glass, would have fallen directly upon the objects which the screen is intended to protect.
[2] The case here imagined is not entirely hypothetical. We examine Mercury and Venus very nearly under the conditions here imagined; for we can obtain only spectroscopic evidence respecting the existence of water on either planet. In the case of Mars we have telescopic evidence, and no one now doubts that the greenish parts of the planet are seas and oceans. But Venus and Mercury are never seen under conditions enabling the observer to determine the colour of various parts of their discs.
I may add that a mistake, somewhat analogous to that which I have described in the cases of an imagined observer of our earth, has been made by some spectroscopists in the case of the planets Jupiter and Saturn. In considering the spectroscopic evidence respecting the condition of these planets’ atmospheres, they have overlooked the circumstance that we can judge only of the condition of the outermost and coolest layers, for the lower layers are concealed from view by the enormous cloud masses, floating, as the telescope shows, in the atmospheric envelopes of the giant planets. Thus the German spectroscopist Vögel argues that because in the spectrum of Jupiter dark lines are seen which are known to belong to the absorption-spectrum of aqueous vapour, the planet’s surface cannot be intensely hot. But Jupiter’s absorption-spectrum belongs to layers of his atmosphere lying far above his surface. We can no more infer the actual temperature of Jupiter’s surface from the temperature of the layers which produce his absorption-spectrum, than a visitor who should view our earth from outer space, observing the low temperature of the air ten or twelve miles above the sea-level, could infer thence the actual temperature of the earth’s surface.
[3] In “Other Worlds than Ours,” I wrote as follows:—“The lines of hydrogen, which are so well marked in the solar spectrum, are not seen in the spectrum of Betelgeux. We are not to conclude from this that hydrogen does not exist in the composition of the star. We know that certain parts of the solar disc, when examined with the spectroscope, do not at all times exhibit the hydrogen lines, or may even present them as bright instead of dark lines. It may well be that in Betelgeux hydrogen exists under such conditions that the amount of light it sends forth is nearly equivalent to the amount it absorbs, in which case its characteristic lines would not be easily discernible. In fact, it is important to notice generally, that while there can be no mistaking the positive evidence afforded by the spectroscope as to the existence of any element in sun or star, the negative evidence supplied by the absence of particular lines is not to be certainly relied upon.”
[4] Dr. Draper remarks here in passing, “I do not think that, in comparisons of the spectra of the elements and sun, enough stress has been laid on the general appearance of lines apart from their mere position; in photographic representations this point is very prominent.”
[5] The word “ignited” may mislead, and indeed is not correctly used here. The oxygen in the solar atmosphere, like the hydrogen, is simply glowing with intensity of heat. No process of combustion is taking place. Ignition, strictly speaking, means the initiation of the process of combustion, and a substance can only be said to be ignited when it has been set burning. The word glowing is preferable; or if reference is made to heat and light combined, then “glowing with intensity of heat” seems the description most likely to be correctly understood.
[6] It would be an interesting experiment, which I would specially recommend to those who, like Dr. Draper, possess instrumental means specially adapted to the inquiry, to ascertain what variations, if any, occur in the solar spectrum when (i.) the central part of the disc alone, and (ii.) the outer part alone, is allowed to transmit light to the spectroscope. The inquiry seems specially suited to the methods of spectral photography pursued by Dr. Draper, and by Dr. Huggins, in this country. Still, I believe interesting results can be obtained even without these special appliances; and I hope before long to employ my own telescope in this department of research.
[7] In 1860, a year of maximum sun-spot frequency, Cambridge won the University boat-race; the year 1865, of minimum sun-spot frequency, marked the middle of a long array of Oxford victories; 1872, the next maximum, marked the middle of a Cambridge series of victories. May we not anticipate that in 1878, the year of minimum spot frequency, Oxford will win? [This prediction made in autumn, 1877, was fulfilled.] I doubt not similar evidence might be obtained about cricket.
[8] It must be understood that this remark relates only to the theory that by close scrutiny of the sun a power of predicting weather peculiarities can be obtained, not to the theory that there may be a cyclic association between sun-spots and the weather. If this association exists, yet no scrutiny of the sun can tell us more than we already know, and it will scarcely be pretended that new solar observatories could give us any better general idea of the progress of the great sun-spot period than we obtain from observatories already in existence, or, indeed, might obtain from the observations of a single amateur telescopist.
I think it quite possible that, from the systematic study of terrestrial relations, the existence of a cyclic association between the great spot period and terrestrial phenomena may be demonstrated, instead of being merely surmised, as at present. By the way, it may be worth noting that a prediction relative to the coming winter [that of 1877–78] has been made on the faith of such association by Professor Piazzi Smyth. It runs as follows:—