THE NEBULÆ OF THE PLEIADES.
(From a photograph taken at the Royal Observatory, Greenwich, December 3, 1899, with an exposure of three hours.)
The reflector of 30 inches' aperture, which acts as a counterpoise to the sheaf of telescopes of the Thompson, is intended for use with the spectroscope, the quality which mirrors possess of bringing all rays, whatever their colour, to the same focus being of great importance for spectroscopic work. But the experiments which have been made with it in celestial photography have proved so extremely successful as to cause the postponement of the recommencement of the spectroscopic researches. Chief amongst these photographs are some good ones of the moon, and more recently some exceedingly fine photographs of the principal nebulæ.
In no department of astronomy has photography brought us such striking results as in regard to the nebulæ. Dr. Roberts' photograph of the great nebula in Andromeda converted the two or three meaningless rifts—which some of the best drawings had shown—into the divisions between concentric rings; and what had appeared a mere shapeless cloud was seen to be a vast symmetrical structure, a great sidereal system in the making. The great nebula in Orion has grown in successive photographs in detail and extent, until we have a large part of the constellation bound together in the convolutions of a single nebula of the most exquisite detail and most amazing complexity. The group of the Pleiades has had a more wonderful record still. Manifestly a single system even to the naked eye, and showing some faint indications of nebulosity in the telescope, the photographs have revealed its principal stars shining out from nebulous masses, in appearance like carded wool, and have shown smaller stars threaded on nebulous lines like pearls upon a string.
Such photographs are, of course, of no utilitarian value, and at present they lead us to no definite scientific conclusions. They lie, therefore, doubly outside the limits of the purely practical, but they attract us by their extreme beauty, and by the amazing difficulty of the problems they suggest. How are these weird masses of gas retained in such complex form over distances which must be reckoned by millions of millions of miles? By what agency are they made to glow so as to be visible to us here? What conceivable condition threads together suns on a line of nebula? What universes are here in the making, or perhaps it may be falling into ruin and decay?
CHAPTER XIII
THE DOUBLE-STAR DEPARTMENT
The foregoing chapters will have shown that though the original purpose of the Observatory has always been kept in view, yet the progress of science has caused many researches to be undertaken which overstep its boundaries. Thus in the present transit room, beside the successive transit instruments we find upon the wall two long thin tubes, labelled respectively Alpha Aquilæ and Alpha Cygni. These were two telescopes set up by Pond for a special purpose. Dr. Brinkley, Royal Astronomer for Ireland, had announced that he had found that several stars shifted their apparent place in the sky in the course of a year, due to the change in the position of the earth from which we view them, by an amount which would show that they were only about six to nine billions of miles distant from us; or, in other words, they showed a parallax of from two to three seconds of arc. Pond was not able to confirm these parallaxes from his observations, and to decide the point he set up these two telescopes, the Alpha Aquilæ telescope being rigidly fixed on the west side of the pier of Troughton's mural circles; the Alpha Cygni telescope on another pier, the one which now forms the base of the pier of the astrographic telescope. Pond's method was to compare the position of these two stars with that of a star almost exactly the same distance from the pole, but at a great distance from it in time of crossing the meridian; in other words, of almost the same declination, but widely different right ascension. The result proved that Brinkley was wrong, and vindicated the delicacy and accuracy of Pond's observations.