existing in our atmosphere. The lines in the spectrum of Mars probably indicate the existence of similar matter in the planet's atmosphere. I suspected that these lines were most distinct in the light from the margin in the planet's disc; but this observation was to some extent uncertain. That these lines were not produced by the portion of the earth's atmosphere through which the light of Mars had passed was shown by the absence of similar lines in the spectrum of the moon, which at the time of observation had a smaller altitude than Mars.

I observed also the spectra of the darker portions of the planet's disc. The spectrum of the dark zone beneath the southern polar spot appeared as a dusky band when compared with the spectra of the adjoining brighter parts of the planet. This fainter spectrum appeared to possess a uniform depth of shade throughout its length. This observation would indicate that the material which forms the darker parts of the planet's surface absorbs all the rays of the spectrum equally. These portions should be therefore neutral, or nearly so, in colour.

I do not now regard the ruddy colour of Mars to be due to an elective absorption, that is, an absorption of certain rays only so as to produce dark lines in the spectrum.

Further, it does not appear to be probable that the ruddy tint which distinguishes Mars has its origin in the planet's atmosphere, for the light reflected from the polar regions is free from colour, though this light has traversed a longer column of atmosphere than the light from the central parts of the disc. It is in the central parts of the disc that the colour is most marked. If indeed the colour be produced by the planet's atmosphere, it must be referred to peculiar conditions of it which exist only in connexion with particular portions of the planetary surface. The evidence we possess at present appears to support the opinion that the planet's distinctive colour has its origin in the material of which some parts of its surface are composed. Mr. Lockyer's observation, that the colour is most intense when the planet's atmosphere is free from clouds, obviously admits of an interpretation in accordance with this view.

This opinion appears to receive support from the photometric observations of Seidel and Zöllner, some of the results of which I will briefly state.

These observations show that Mars resembles the moon in the anomalous amount of variation of the light reflected from it as it increases and decreases in phase; also in the greater brilliancy of the marginal portions of its disc. Further, Zöllner has found that the albedo of Mars, that is, the mean reflective power of the different parts of its disc, is not more than about onehalf greater than that of the lunar surface. Now these optical characters are in accordance with telescopic observation, that in the case of Mars the light is reflected almost entirely from the true surface of the planet. Jupiter and Saturn, the light from which has evidently come from an envelope of clouds, are, on the contrary, less bright at the margin than at the central part of the disc. These planets have an albedo, severally, about four and three times greater than that of the moon.1

1

The anomalous degradation in the brightness of the moon at the phases on either side of the full, as well as the greater brilliancy of the limb, may be accounted for by the supposition of inequalities on its surface, and also by a partly regular reflective property of its superficial rocks. Zöllner has shown that if these phenomena be assumed empirically to be due to inequalities, then the angle of mean elevation of these inequalities must be taken as 52°. On the same hypothesis the more rapid changes of Mars would require an angle of 76°.2

It appears to be highly probable that the conditions of surface which give rise to these phenomena are common to the moon and to Mars. The considerations referred to in a former paragraph suggest that these superficial conditions represent peculiarities which exist at the true surface of the planet. In this connexion it is of importance to remark that the darker parts of the disc of Mars gradually disappear, and the coloured portions lose their distinctive ruddy tint as they approach the limb.

1 Photometrische Untersuchungen, von Dr. J. C. Zöllner, Leipzig, 1865. 2 Ibid. pp. 113, 128.

The observations of Sir John Herschel1 and Professor G. Bond2 give a mean reflective power to the moon's surface, similar to that from a grey, weathered sandstone rock.” Zöllner has confirmed this statement. According to him,——

From this table it appears that Mars takes in for its own use 7328 of the energy which it receives as light. Jupiter's cloudy atmosphere, nearly as brilliant as white paper, rejects more than six-tenths of the light which falls upon it. Therefore, less than four-tenths of the light which this distant planet receives is alone available for the purposes of its economy.

The photographic researches of Mr. De La Rue and others show that the rays of high refrangibility, which are specially powerful in producing chemical action, are similarly affected.3 At present we know nothing of the reflective power of the planets for those rays of slower vibration which we call heat.

1 Outlines of Astronomy, p. 272.

"On the Light of the Moon and Jupiter," Memoirs Amer. Academy, vol. iii. p. 222. In the same Memoir Prof. G. Bond estimates the albedo of Jupiter to be greater than unity. This estimate would require the admission that Jupiter shines in part by native light. (Ibid. p. 284.)

Prof. G. Bond states that "the moon, if the constitution of its surface resembled that of Jupiter, would photograph in one-fourteenth of the time it actually requires." (Ibid. p. 223.)

APPENDIX C.

ON THE SPECTRA OF VARIABLE STARS.

T

"The spectrum of y Cassiopeia appears to be in some respects at least analogous to that of Coronæ. In addition to the bright line near the boundary of the green and blue observed by Father Secchi, there is a line of equal brilliancy in the red, and some dark lines of absorption. The two bright lines are narrow and defined, but not very brilliant. Micrometrical measures made by Mr. Huggins of these lines show that they are doubtless coincident in position with Fraunhofer's c and F, and with two of the bright lines of luminous hydrogen. In these stars part of the light must be emitted by gas intensely heated, though not necessarily in a state of combustion. The nearly uniform light of y Cassiopeiæ suggests that the luminous hydrogen of this star forms a normal part of its photosphere."—Notices, Royal Astronomical Society, vol. xxvii. p. 131.

Mira Cali, which gives a spectrum apparently identical, or nearly so, with a Orionis, was examined when at its maximum brilliancy, and on several subsequent occasions, after it had commenced its downward course. At the time the star was waning in brightness there was thought to be an appearance of greater intensity in several of the groups, but a continued series of observations is desirable before any opinion is hazarded as to the cause of the variation in brightness which has procured for this object the title of Wonderful.' At Mr. Baxendell's request the variable ρ Coronæ was examined when at its maximum, but without any successful result. . . . Mr. Huggins has confirmed the observation of MM. Wolf and Raget so far as to the presence of bright lines in the three small stars described by them. He has not determined the positions of these lines."-Ibid. vol. xxviii. p. 87.

APPENDIX D.

FURTHER OBSERVATIONS ON THE SPECTRA OF SOME OF THE STARS AND NEBULÆ, WITH AN ATTEMPT TO DETERMINE THEREFROM WHETHER THESE BODIES ARE MOVING TOWARDS OR FROM THE EARTH; ALSO OBSERVATIONS ON THE SPECTRA OF THE SUN AND OF COMET II. 1868.1

BY WILLIAM HUGGINS, ESQ. F.R.S.

§ I. Introduction.

In a paper "On the Spectra of some of the Fixed Stars," by myself and Dr. W. A. Miller, Treas. R.S., we gave an account of the method by which we had succeeded during the years 1862 and 1863 in making trustworthy simultaneous comparisons of the bright lines of terrestrial substances with the dark lines in the spectra of some of the fixed stars. We were at the time fully aware that these direct comparisons were not only of value for the more immediate purpose for which they had been undertaken, namely, to obtain information of the chemical constitution. of the investing atmospheres of the stars, but that they might also possibly serve to tell us something of the motions of the stars relatively to our system. If the stars were moving towards or from the earth, their motion, compounded with the earth's motion, would alter to an observer on the earth the refrangibility of the light emitted by them, and consequently the lines of terrestrial substances would no longer coincide in position in the spectrum with the dark lines produced by the absorption of the vapours of the same substances existing in the stars.

The apparatus employed by us was furnished with two prisms of dense flint glass, each with a refracting angle of 60°, and permitted the comparisons to be made with so much accuracy 2 Ibid. 1864, p. 413.

1 Phil. Trans. 1868, p. 529.