Part 13 (1/2)

[Footnote 288: _Annuaire_, 1836, p. 233.]

[Footnote 289: _Cosmos_, vol. i., p. 90, _note_ (Otte's trans.).]

[Footnote 290: Herschel, _Outlines of Astronomy_, p. 399, 9th ed.]

[Footnote 291: _Outlines_, p. 398.]

[Footnote 292: Boguslawski calculated that it extended on the 21st of March to 581 millions.--_Report. Brit. a.s.s._, 1845, p. 89.]

[Footnote 293: _Comptes Rendus_, t. xvi., p. 919.]

[Footnote 294: _Observatory_, vol. xxiv., p. 167; Astr. Nach., No.

3,320.]

[Footnote 295: Piazzi noticed a considerable increase of l.u.s.tre in a very faint star of the twelfth magnitude viewed through a comet. Madler, _Reden_, etc., p. 248, _note_.]

[Footnote 296: _Astr. Jahrbuch_, 1828, p. 151.]

[Footnote 297: Madler, _Gesch. d. Astr._, Bd. ii., p. 412.]

[Footnote 298: _Recueil de l'Ac. Imp. de St. Petersbourg_, 1835, p.

143.]

[Footnote 299: Guillemin's _World of Comets_, trans, by J. Glaisher, p.

294, _note_.]

[Footnote 300: _Month. Not._, vol. viii., p. 9.]

[Footnote 301: A real, though only partial stoppage of light seems indicated by Herschel's observations on the comet of 1807. Stars seen through the tail, October 18, lost much of their l.u.s.tre. One near the head was only faintly visible by glimpses. _Phil. Trans._, vol. xcvii., p. 153.]

[Footnote 302: Arago, _Annuaire_, 1832, p. 205.]

[Footnote 303: _Ibid._, 1891, p. 290.]

[Footnote 304: Viz., Encke's, Biela's, Faye's, and Brorsen's.]

CHAPTER VI

_INSTRUMENTAL ADVANCES_

It is impossible to follow with intelligent interest the course of astronomical discovery without feeling some curiosity as to the means by which such surpa.s.sing results have been secured. Indeed, the bare acquaintance with _what_ has been achieved, without any corresponding knowledge of _how_ it has been achieved, supplies food for barren wonder rather than for fruitful and profitable thought. Ideas advance most readily along the solid ground of practical reality, and often find true sublimity while laying aside empty marvels. Progress is the result, not so much of sudden flights of genius, as of sustained, patient, often commonplace endeavour; and the true lesson of scientific history lies in the close connection which it discloses between the most brilliant developments of knowledge and the faithful accomplishment of his daily task by each individual thinker and worker.

It would be easy to fill a volume with the detailed account of the long succession of optical and mechanical improvements by means of which the observation of the heavens has been brought to its present degree of perfection; but we must here content ourselves with a summary sketch of the chief amongst them. The first place in our consideration is naturally claimed by the telescope.

This marvellous instrument, we need hardly remind our readers, is of two distinct kinds--that in which light is gathered together into a focus by _refraction_, and that in which the same end is attained by _reflection_. The image formed is in each case viewed through a magnifying lens, or combination of lenses, called the eye-piece. Not for above a century after the ”optic gla.s.ses” invented or stumbled upon by the spectacle-maker of Middelburg (1608) had become diffused over Europe, did the reflecting telescope come, even in England, the place of its birth, into general use. Its principle (a sufficiently obvious one) had indeed been suggested by Mersenne as early as 1639;[305] James Gregory in 1663[306] described in detail a mode of embodying that principle in a practical shape; and Newton, adopting an original system of construction, actually produced in 1668 a tiny speculum, one inch across, by means of which the apparent distance of objects was reduced thirty-nine times. Nevertheless, the exorbitantly long tubeless refractors, introduced by Huygens, maintained their reputation until Hadley exhibited to the Royal Society, January 12, 1721,[307] a reflector of six inches aperture, and sixty-two in focal length, which rivalled in performance, and of course indefinitely surpa.s.sed in manageability, one of the ”aerial” kind of 123 feet.

The concave-mirror system now gained a decided ascendant, and was brought to unexampled perfection by James Short of Edinburgh during the years 1732-68. Its resources were, however, first fully developed by William Herschel. The energy and inventiveness of this extraordinary man marked an epoch wherever they were applied. His ardent desire to measure and gauge the stupendous array of worlds which his specula revealed to him, made him continually intent upon adding to their ”s.p.a.ce-penetrating power” by increasing their light-gathering surface. These, as he was the first to explain,[308] are in a constant proportion one to the other.

For a telescope with twice the linear aperture of another will collect four times as much light, and will consequently disclose an object four times as faint as could be seen with the first, or, what comes to the same, an object equally bright at twice the distance. In other words, it will possess double the s.p.a.ce-penetrating power of the smaller instrument. Herschel's great mirrors--the first examples of the giant telescopes of modern times--were then primarily engines for extending the bounds of the visible universe; and from the sublimity of this ”final cause” was derived the vivid enthusiasm which animated his efforts to success.

It seems probable that the seven-foot telescope constructed by him in 1775--that is within little more than a year after his experiments in shaping and polis.h.i.+ng metal had begun--already exceeded in effective power any work by an earlier optician; and both his skill and his ambition rapidly developed. His efforts culminated, after mirrors of ten, twenty, and thirty feet focal length had successively left his hands, in the gigantic forty-foot, completed August 28, 1789. It was the first reflector in which only a single mirror was employed. In the ”Gregorian” form, the focussed rays are, by a second reflection from a small concave[309] mirror, thrown _straight back_ through a central aperture in the larger one, behind which the eye-piece is fixed. The object under examination is thus seen in the natural direction. The ”Newtonian,” on the other hand, shows the object in a line of sight at right angles to the true one, the light collected by the speculum being diverted to one side of the tube by the interposition of a small plane mirror, situated at an angle of 45 to the axis of the instrument. Upon these two systems Herschel worked until 1787, when, becoming convinced of the supreme importance of economising light (necessarily wasted by the second reflection), he laid aside the small mirror of his forty-foot then in course of construction, and turned it into a ”front-view”

reflector. This was done--according to the plan proposed by Lemaire in 1732--by slightly inclining the speculum so as to enable the image formed by it to be viewed with an eye-gla.s.s fixed at the upper margin of the tube. The observer thus stood with his back turned to the object he was engaged in scrutinising.