These numbers represent not the apparent brightness only, but the true brightness of these luminous masses, except so far as it may have been diminished by a possible power of extinction existing in cosmical space, and by the absorption of our atmosphere. It is obvious that similar observations, made at considerable intervals of time, may show whether the light of these objects is undergoing increase or diminution, or is subject to a periodic variation. If the dumb-bell nebula, the feeble light of which is not more than one twenty-thousandth part of that of a candle, be in accordance with popular theory a sun-germ, then it is scarcely possible to put in an intelligble form the enormous number of times by which its light must increase before this faint nebula, feebler now in its glimmering than a rushlight, can rival the dazzling splendour of our sun.

MEASURES OF THE NEBULE.

Some of the nebula are sufficiently defined in outline to admit of accurate measurement. By means of a series of micrometric observations, it will be possible to ascertain whether any considerable alteration in size takes place in nebulæ.

METEORS.

Mr. Alexander Herschel has recently succeeded in subjecting another order of the heavenly bodies to prismatic analysis. He has obtained the spectrum of a bright meteor, and also the spectra of some of the trains which meteors leave behind them. A remarkable result of his observations appears to be that sodium in the state of luminous vapour is present in the trains of most

meteors.

CONCLUSION.

In conclusion, the new knowledge that has been gained from these observations with the prism may be summed up as follows:

1. All the brighter stars, at least, have a structure analogous to that of the

sun.

2. The stars contain material elements common to the sun and earth. 3. The colours of the stars have their origin in the chemical constitution of the atmospheres which surround them.

4. The changes in brightness of some of the variable stars are attended with changes in the lines of absorption of their spectra.

5. The phenomena of the star in Corona appear to show that in this object at least great physical changes are in operation.

6. There exist in the heavens true nebulæ. luminous gas.

7. A part of the light of comets is self-luminous.

These objects consist of

8. The bright points of the star-clusters may not be in all cases stars of the same order as the separate bright stars.

It may be asked what cosmical theory of the origin and relations of the heavenly bodies do these new facts suggest? It would be easy to speculate, but it appears to me that it would not be philosophical to dogmatize at present on a subject of which we know so very little. Our views of the universe are undergoing important changes; let us wait for more facts with minds unfettered by any dogmatic theory, and therefore free to receive the obvious teaching, whatever it may be, of new observations.

Star differs from star in glory, each nebula and each cluster has its own special features, doubtless in wisdom and for high and important purposes the Creator has made them all.

On some further Results of Spectrum Analysis as applied to the Heavenly Bodies. By WILLIAM HUGGINS, F.R.S., Hon. Sec. to the Royal Astronomical Society*.

[Plate V.]

Two years ago, at the Meeting of the British Association at Nottingham, I had the honour to give, in an evening discourse, a summary of the results of Spectrum Analysis as applied to the heavenly bodies, which had been obtained partly by myself and partly as the conjoint work of myself and Dr. W. Allen Miller, Treas. and V.P.R.S.

I beg now to offer to the Association a brief account of some of the principal results of the observations which I have made since August 1866. These observations may be arranged according to the classes of the heavenly bodies to which they relate.

§ I. On the Fixed Stars.

§ II. On the Nebula.

§ III. On the Light of Comets.

§ IV. On the Spectra of Sunspots.

SV. On the Planets.

§ I. ON THE FIXED STARS.

A. Observations to determine whether the Stars are moving towards or from the Earth.

The determination of the proper motions of stars from observations of their angular motions among the stars apparently near them, gives to us information of that part only of their motion which is at right angles to a ray of light coming from the star to the observer. It would be a rare accident only if the motion thus obtained represented the whole of their motion. The other part of the star's motion, namely, the movement of the star in the direction of the visual ray towards or from the earth, seemed to be beyond the reach of our means of observation; for photometric estimation of the increase or diminution of the light of the star was obviously too coarse and uncertain a method for so delicate an investigation.

Now it is precisely information on this point which appeared to be inaccessible to us, which has been brought within our reach by means of observations with the prism. Supposing waves to be coming in upon the shore, a ship leaving the harbour would encounter a larger number of these waves in a given time than a ship would at anchor; and further, the increased velocity of succession of the waves which would strike its prow could be determined if the velocity of the ship and that of the waves were known. Conversely, if the period and the velocity of the waves had been ascertained, the captain, by counting the number of waves which met the ship in a given interval of time, could determine therefrom the motion of his vessel. A little consideration will make it evident that a similar effect would take place if the vessel were at rest, and the source of wave-motion were supposed to approach or recede from the vessel; in this case the velocity of the source of wave-motion could be determined, if the initial period of the waves and the velocity of their propagation were known. This illustration sets forth the principles on which is founded the method of investigation which is now to be described.

The idea that a change of period in luminous or sonorous waves would arise in consequence of a motion of the observer, or of the source of the light,

* A communication ordered to be printed in cætenso.

or of the sound, towards or from each other, is due to Doppler. In 1841 Doppler showed that since the impression which is received by the eye or the ear does not depend upon the intrinsic strength and period of the waves of light and of sound, but is determined by the interval of time in which they fall upon the organ of the observer, it follows that the colour and intensity of an impression of light and the pitch and strength of a sound. will be altered by a motion of the source of the light or of the sound, or by a motion of the observer, towards or from each other*.

Doppler then went wrong; for he sought by these considerations to account for the remarkable difference of colour which some of the binary stars present, and for some other phenomena of the heavenly bodies. Now it is obvious that if a star could be conceived to be moving with a velocity sufficiently great to alter its colour sensibly to the eye, still no change of colour would be perceived, for the reason that, beyond the visible spectrum, at both extremities there exists a store of invisible waves which would be at the same time exalted or degraded into visibility to take the place of the waves which had been raised or lowered in refrangibility by the star's motion. No change of colour, therefore, could take place until the whole of these invisible waves of force had been used up, which would only be the case when the relative motion of the source of light and of the observer was several times greater than that of light.

It is obvious from these considerations that this method of research could afford us information of the motion of the star only in the case in which we knew the period of the light at the time of its emission from the star; for then a comparison of this initial period with that observed at the earth would show the exact amount of the change of refrangibility due to the relative motions of the observer and the star, and as the earth's motions are known, the motion of the star could be determined.

Now this one essential condition, namely, the knowledge of the period of the light when emitted by the star, is fulfilled by spectrum analysis. When we learn the existence of a terrestrial substance in a star, we have the means of knowing the initial refrangibility of the dark lines in the star's spectrum, which are due to the absorption of the vapour of this substance. It may be thought that if the lines in the spectra of the stars are subject to an unknown amount of displacement from the cause we have now under consideration, it would not be possible to make use of these lines to learn the star's chemical constitution. This objection, however, does not obtain; for the amount by which the lines would be displaced by any velocity we could with probability assign to the stars, would be too small to be even perceived in the spectroscopes which had hitherto been applied to the heavenly bodies. For example, a velocity ten times greater than that of the earth in its orbit would cause a line to move through a space in the spectrum about as great as that which separates the components of the double line D of the solar spectrum. Besides this consideration, the trustworthiness of the results obtained by myself and Dr. Miller in our joint researches, was not allowed to rest upon the position of a single line, but upon the coincidence in general character as well as in position of a group of several lines. At the time, indeed, when we made our observations, we were fully aware that these direct comparisons were not only of value for the determination of the chemical constitution of the stars, but that they might tell us something of the motions

Ueber das farbige Licht der Doppelsterne und einiger anderer Gestirne des Himmels," Böhm. Gesell. Abh. ii. 1842-44, s. 465.