^{page 371 note *} It would not be right to ignore that both England and Ireland have object-glasses of 12 inches in diameter; that the Russian Observatory of Pulkova has possessed for a quarter of a century, and used with great profit to exact astronomy, an object-glass of 15 inches in diameter; and that, in addition to Paris, the astronomers of the United States have now in employment one of the same size, and also one even of 18 inches in diameter, the normal size for a new generation, with similar advantageous results.

^{page 373 note *} This high power could be kept constantly on, without inconvenience when first picking up any small star, owing to the luxurious furnishing of finders to the large telescope, for it had no less than three such appendages; whereof the first had a 4·5-inch object-glass, and a magnifying power of 56 times; the second, a 2·2-inch object-glass, and magnifying power of 28 times; and the third, a 1·7-inch object-glass, and magnifying power of 17 times.

^{page 378 note *} The reading of the equator on the position circle of the Elchies micrometer was obtained every observing night, in the usual manner, by equatorial stars being made to “thread” along a wire. The value of the micrometer screw in seconds of space was derived from transits of the polestar, measured with a sidereal pocket chronometer, kindly lent by Dr Lee of Hartwell to the Edinburgh Royal Observatory for many years, and brought northwards on this occasion.

^{page 381 note *} The numbers in simple parenthesis after an Elchies observation, indicate the number of nights on which it was observed in any particular element; but when a w is prefixed to the number, it indicates the estimated weight concluded on the principles mentioned in page 376. The original authorities referred to for numerical observations, are,—

“Mensuræ Micrometricæ” of F. G. W. von Struve, quoted as Σ, when observing with the 9-inch, and σ with the 3·6-inch Dorpat Equatorial.

“Cape of Good Hope Observations,” by Sir J. F. W. Herschel, quoted as H².

“Cycle,” by Admiral W. H. Smyth, 1844.

“Speculum Hartwellianum,” by the same, 1860.

Memoirs and Monthly Proceedings of Royal Astronomical Society; Astronomisches, Nachrichten, by Dr A. Auvers; and Edinburgh Astronomical Observations, vol. xii., for Guajara and Alta Vista observations in Teneriffe. Other references to Sir William Herschel, as H¹, and Sir John Herschel and Sir James South, as H² and S, are extracted either from the “Mensuræ Micrometricæ,” or the “Cycle”

^{page 387 note *} Astronomische Nachrichten, No. 1393, p. 10.

^{page 396 note *} It was not M. Struve, nor indeed any astronomer whatever, who was thus referred to.

^{page 400 note *} Struve's note is as follows:—

“Colorum diversitas ex singulorum dierum consensu indubia est. Probe notandum est has colorum notationes sine ullo præjudicio esse factas, cum in tot stellis duplicibus observandis occupatus, nunquam recordarer, quid peculiaris in hoc illave notaverim. Eadem colorum relatio etiam a H² et S. est reperta, scilicet priorem esse flavescentem, posteriorem non. Eadem denique in observationibus per instrumenta meridiana sæpius annotavi. Insignis est differentia luminis in stelli ejusdem proxime splendoris.”

^{page 415 note *} The instances in Correction-Table II., p. 377, having been further examined for the often reputed effect of absolute position angle in biasing more or less the estimation of any observer in both position and distance, have given some indications of a small correction of that nature, varying with the position, being required as below; the distances having been first corrected for a probable residual error, both of run of micrometer and method of measuring, amounting at a distance of 5″ to + ·04″, at 10″ to + ·01″, at 15″ to −·03″, at 20″ to −·08″, and at 25″ to −·13″.

These corrections, though regular, are so small, compared to the ordinary error of individual observations, that they have not yet been applied in any case.

^{page 416 note *} Some further inquiries may also be subserved by the Elchies colour observations; which, if unusually few in number, have been more than usually attended to touching the character of the double stars concerned, or the difference of the distance of their members from our system.

There is undoubtedly an ether filling space, say most scientific men; well then, if so, what is its colour by transmitted light? Star observations are peculiarly adapted to this end, and the colours which we recognise in all stars may partly belong to this medium, whose colour, too, and composition may be varied in particular regions. First, then, let us ascertain if there is any constant feature of colour dependent on distance. Now the two nearest well-determined stellar systems are those of α Centauri and 61 Cygni; and Sir J. Herschel has remarked their strong yellow colour, stronger in the small, than the large, component in either case; if, then, those stars are really white, but appear yellow to us, they give in so far a quadruple proof of the medium which extends between them and ourselves, being yellow by transmitted light; by no means an extraordinary result, if, according to recent mathematicians, the atmosphere of the earth thins away and extends indefinitely into the planetary and stellar spaces.

But when we pursue our inquiries further still, and beyond the limits of sensible stellar parallax, and merely employ the broad assumption that the smaller that stars do appear, the further off they must be on the whole,—then arises the anti-terrestrial result, that the greater the depth of the distant medium through which stars are seen, the bluer they become; for all very small stars are bluish. The smaller members also of most double stars, whether optical or physical, and of unknown distance from us, are, as a whole, bluer than the larger members; appearing thereby to imply, though there may be other concurring causes to be mentioned, that the medium filling the more distant realms of space is in a positively different physical condition from that in which we are moving at present under the control and guardianship of our sun.

It is even possible that there are more restricted regions of speciality still, and that floating cosmical motes may exist in interstellar space, capable of playing very peculiar parts on the light transmitted through them from more distant bodies. Cosmical clouds have been suggested by both the Herschels to explain observed changes in telescopic stars and nebula; and such clouds, not very far too from our own system, may be strongly suspected, some might even be inclined to say are found, to exist, from the many variable stars whose periods have been ascertained to be almost exactly 365 days. (See Spec. Hart., p. 269.)

On the other hand, our case of 95 Herculis, even if there had not been any thing else of the same order, establishes the point, of colours and changes of colours peculiar to, or arising in, the stars themselves. That instance forms an extreme case; but the forces at work there are probably in existence elsewhere as well, and not improbably in our own sun, and even in our own earth. Let me explain the hasty generalisation, which, in the absence of almost anything else, may at least incite to further observation. The changing tints of the members of 95 Herculis are eminently auroral, i.e. of the character of an Aurora Borealis when its displays are intense, such as we see them in the northern sky once in half a century only, and when fiery pink and vivid green streamers alarm the country; and they are auroral also in their indications of fitful flashes and pulses, like the changes of lustre in periods of a few seconds noticed by Mr Norman Pogson in the star U Geminorum (Spec. Hart. p. 107). Again, the pink prominences seen at total eclipses around our own sun, and at times or in parts varying to other bluish colours, as established by Otto Struve, with the approbation of Mr Airy in 1860, have likewise been analogised by Mr Balfour Stuart to terrestrial auroras. Something then of the same character that produces the immense effects observed on the orbs of 95 Herculis, may, and apparently does, exist on the surface of our own sun; following too, in its luminous and coloured manifestations, such active and impetuous changes of short period (for the eclipse red prominences have never been seen twice alike), that some notable maxima, from the occurrence of secular periods, capable of reacting on the climate of our earth, may at times be looked for.

Thus the subject of the colours of the stars becomes one, not only of great astronomical, but also of terrestrial, interest and extraordinary complication; and it is possible that spectrum analysis combined with eye estimations, may enable us to separate the colour produced by transmission through intervening cosmical clouds from the colour of the star itself, by reason of the small difference in the dark lines of the spectrum effected by any colouring material at a low temperature.