of ordinary, or what we may call annual aberration, is to make them appear to describe little ellipses, the semi-axes of which depend upon the ratio of the velocity of light to the velocity of the earth in its orbit. But the sun's orbital movement being conducted, so far as experience yet goes, in one direction, the aberration due to it is in one direction too, and is hence constant, and for the present beyond the reach of observation. It is, however, constant only so long as the movement producing it remains sensibly so. As the latter changes, it will change too, and may in this way be brought within the domain of human cognisance. For upon the acceleration, retardation, or deflection of the sun's movement systematic displacements among the stars should ensue, the nature of which would at once betray their origin.

The total amount of this secular aberration may be roughly stated as one second of arc for every mile per second of the sun's velocity. Hence, stars 90° from the solar apex are pushed forward towards it by perhaps 15", the effect upon other stars diminishing with the sines of their distances on the sphere from the same point. These aberrational can be distinguished from the parallactic displacements flowing from the same source, by their indifference to remoteness in space. Stars far and near, bright and faint, swift-moving and tardy, are equally affected by them. But while it is quite certain that visual disturbances of this kind take place, and to an extent possibly greatly in excess of that above assigned to them, their interest must for a long time remain purely theoretical. Indeed, it may well be that the modifications rendering them sensible and instructive will proceed with such exorbitant slowness that not even astronomical patience will avail to unmask them.

We do not know the plane of the sun's orbit-only the direction of one line in it. And that line, pointing towards the constellation Hercules, makes an angle of about 60° with the sun's equator. Thus, the solar movements of rotation and translation would seem to be unrelated one to the other; and the same remark applies to the planetary revolutions conducted, on the whole, along levels of space differing very

little from that of the greater globe's axial movement. Our whole system is then driven obliquely upward by a power which, taking no apparent account of its domestic economy, owned doubtless an origin totally disconnected from that of gyrations given, through its influence, the helicoidal shape illustrated in fig. 48.

The sun's course, being inclined some twenty-eight degrees to the central plane of the Milky Way, is at present gradually removing it from that stupendous collection. This, however, does not necessarily imply real separateness. The movement of withdrawal actually progress

ing may be only temporary, in the sense that, after countless millions of years, it will be compensated by a return movement of approach. It is difficult to conceive that the combined attractions of the galactic myriads can, in the long run, be resisted. The most probable supposition as to the situation of the centre of force swaying our system, is that it lies somewhere in the cloudy zone which so enhances the mysterious beauty of our

Earth's axis

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Sun's way

Earth's Helicoidal path in space ___

skies. If the orbit we are pur- FIG. 48.-The Earth's Motion in suing be approximately circular,

Space.

then its centre must be divided by a quadrant of the sphere from the apex-it must lie somewhere on a great circle of which the apex and anti-apex are the poles. Now this great circle cuts the Milky Way at two opposite points in Perseus and Hydra, and there, accordingly, two alternative centres of the solar motion might be looked for. Argelander chose as the more promising position the spot marked by the great cluster in the sword handle of Perseus; but the conjecture made no pretension to scientific authority, and the postulate upon which it was based, of the sun's

Mémoires présentés, St. Pétersbourg, t. iii. p. 302.

path being at all nearly circular, is in truth of a highly precarious nature.

We are even ignorant whether the translation of our system towards the constellation Hercules represents a primary or a secondary order of stellar revolution. It perhaps merely indicates the interstitial movement appertaining to the sun as a member of a restricted group of stars, the common transport of which proceeds undetected in a totally different direction. Hence the possibility, suggested by Herschel, of the presence of a higher kind of systematic parallax than that revealed in the drift of the brightest stars.' It has, however, yet to be discovered, and time is short for the investigations which its discovery would demand.

Progress is here only possible through careful and minute study of the residual movements of the stars-of the movements, that is to say, which remain after the general perspective effect of the sun's motion has been subtracted, and which belong, accordingly, to their individual selves. The questions connected with them which most immediately present themselves are these: Has the sun companions on its journey, or does it travel alone? and, Are real stellar displacements governed by any obvious law?

The great multitude of the stars are, to all appearance, indifferent to the transport of our system. They have clearly no share in it. Just because they stand aloof, and act as indicators of the way, its progress becomes sensible to us. For motion is not alone undiscoverable, it is even unimaginable without some fixed point of reference. We cannot, however, as yet pronounce with certainty against the existence of a particular dynamical bond connecting the sun with some few of the stars, which together with it form a company associated by subjection to identical influences, and engaged with it on the same journey through space. As to the criteria by which such associated stars, if present, can be discriminated from the rest, something will be said in the next chapter.

There, too, we will consider what answer should be given to our second query. A great deal depends upon it as regards Phil. Trans. vol. lxxiii. pp. 1276-7.

our conception of the sidereal universe. Nay, the result of inquiry upon the point has a vital bearing upon the subject we have just attempted, however inadequately, to deal with. For the assumption that the absolute movements of the stars have no preference for one direction over another forms the basis of all investigations hitherto conducted into the translatory advance of the solar system. The little fabric of laboriously acquired knowledge regarding it at once crumbles if that basis has to be removed. In all investigations of the sun's movement, the movements of the stars have been regarded as casual irregularities; should they prove to be in any visible degree systematic, the mode of treatment adopted (and there is no other at present open to us) becomes invalid, and its results null and void. The point is then of singular interest; and the evidence bearing upon it deserves our utmost attention.

CHAPTER XXII.

THE PROPER MOTIONS OF THE STARS.

WHEN the relative positions of the stars are compared at considerable intervals of time, they are in many cases found to have undergone small, but unmistakable changes of a seemingly capricious character. These are termed proper motions,' to distinguish them from merely nominal shiftings due to the slow variation of the points of reference which serve to define the places of all the heavenly bodies as seen projected on the inner surface of an imaginary concave sphere. Proper motions are by no means easy to get at. Only from the most delicate observations, and with stringent precautions for bringing those at distant dates under precisely similar conditions, can they be elicited with satisfactory accuracy. Otherwise, some trifling systematic discrepancies in the compared catalogues, or accidental errors of computation, might pass for genuine effects of movement, with disastrous influence upon sidereal investigations. Hence, proper motions cannot generally be regarded as established unless, in addition to the terminal observations showing a sufficiently marked change of place in the course of thirty, fifty, or one hundred years, at least one intermediate observation is at hand to prove that the suspected motion has proceeded uniformly in the same direction, and is accordingly not the creation of personal or instrumental inaccuracy.

Although not one of the millions of telescopic stars can, with any show of reason, be supposed at rest, less than five thousand of the stellar army are at present securely credited with measurable and progressive displacements. Many of these, including nearly all the lucide of the northern hemisphere, were observed by Bradley between 1750 and 1762;