but a few years ago it exhibited remarkable changes." We thus see that Betelgeuse is a variable star, and it must have passed in its different variations between the limits of extreme brilliancy, in which the lines of hydrogen appear bright, and that of a less brilliant stage, in which they appear dark,--that is, as absorption bands. It has thus, in fact, run the gamut, so to speak, of color changes, and now occupies an intermediate position in the scale. In his article "Star unto Star," the same writer says, "On this view we may fairly assume that the darkness of the hydrogen lines is a characteristic of stars at a much higher temperature than our sun and suns of the same class." We have already seen that the spectra of stars of the fourth type--Appleton's Cyclopædia, "Spectrum Analysis"--"show the lines of hydrogen bright instead of dark, as though surrounded by hydrogen glowing with a heat more intense than that of the central orb itself." Professor Dunkin says, in his work "The Midnight Sky," "One of the conclusions drawn by Kirchhoff from these experiments is that each incandescent gas weakens, by absorption, rays of the same degree of refrangibility as those it emits; or, in other words, that the spectrum of each incandescent gas is reversed when this gas is traversed by rays of the same refrangibility emanating from an intensely luminous source which gives of itself a continuous spectrum like that of the sun." ... "The third division, including Betelgeuse, Antares, Alpha Herculis, and others of like color, seems to be affected by something peculiar in their physical composition, as if their photospheres contained a quantity of gas at a lower temperature than usual. The stars in this class have generally a ruddy tint, probably owing to their light having undergone some modification while passing through an absorbing atmosphere.... A great number of the stars in the third division are variable in their lustre." We may therefore readily conclude that midway between the inverted lines which constitute the dark absorption bands and the faint spectra which show the bright lines of hydrogen direct there must be an atmosphere of glowing hydrogen superposed upon a deeper one in such proportion that it will not reveal its presence in the spectroscope at all; for when the dark and light bands, which occupy precisely the same position in the spectrum, are of approximately equal intensity the result will obviously be the neutralization of both. That among a myriad suns, some with dark hydrogen lines and some with bright, there should occur occasionally an example corresponding to this point of divergence, and especially among variable stars, is not only to be expected, but is, in fact, confirmatory of the general hypothesis itself. It is an exception which emphatically proves the rule, when we can trace the operative cause which has produced it. CHAPTER VI. THE PHENOMENA OF THE STARS.