for avoiding certain difficulties, mechanical and optical, which had hitherto prevented large reflecting telescopes from being used; and as the range of his knowledge of optics extended, he began to grasp the possibility of improvements in telescopes which should confer on them powers beyond the wildest dreams of former astronomers.
In Miss Herschel's memoir, as in all books dealing with astronomers and their doings, we hear constantly of Gregorian telescopes, Newtonian telescopes, Galilean telescopes. In works of later date we read of Herschelian telescopes; but naturally in works written for professed astronomers no one ever thinks it worth while to say in what the peculiarities of these instruments consist. It may therefore be mentioned here that all telescopes are modifications of two great types: refracting and reflecting. Refracting telescopes consist of a double convex lens, called the object-glass, to enlarge the object viewed, and a smaller double concave lens, or eye-piece, which is used as a microscope to examine the image formed by the object-glass. The common opera-glass is a telescope of this description. It is called after Galileo, the Florentine astronomer. Galileo is said to have received some casual information on the subject from a German whom he met at Genoa in 1609. He was able, after some experiments, to make a telescope which magnified no less than three times! He subsequently made one magnifying thirty-two times. The telescopes used by Huygens and Cassini did not exceed at their highest power one hundred and fifty times. Auzont, who constructed a telescope of three hundred feet focal length, to obviate chromatic aberration (a difficulty on which we do not propose to dwell, as in Herschel's time it had been substantially overcome), applied to his huge and unwieldy instrument a magnifying power of six hundred times.[1]
Reflecting telescopes consist of a concave mirror presented to the object viewed. In the focus of the curve formed by this mirror, and consequently in the spot where all the rays reflected by the mirror converge to a point, a smaller mirror is fixed, facing the first; and the image of the object looked at, after being magnified by the first mirror and concentrated on the small one, is examined by a lens or microscope in the same way that the eye-piece of the Galilean telescope examines the enlarged image made by the object-glass. It must be remembered that the focus, or focal point of a concave mirror, is the centre of the curve presented by such mirror. To make this clear, let us recur to first principles. A sphere is generated by the revolution of a circle round its axis; we may see it by spinning a half-crown on the table. Suppose, then, a circle of three inches radius so rotated, the result would be a sphere of six inches in diameter. If that sphere were formed of glass, and you cut out from any part of it a circular disc with a radius of one inch, you would have a concave glass, like a watch-glass, which would be described as being of two inches diameter, and three inches focal length.
We have said that the small mirror is placed in the focus of the curve of the large mirror, we did so to avoid distracting attention from the principle on which it is constructed; but the peculiarity of the Gregorian[2] telescope is that the small mirror is concave, and is fixed beyond the focal point of the larger reflector; while in the instrument invented by Cassegrain,[3] and called after him, the small mirror is convex, and is placed within the focal distance.
In both these instruments the reflector is perforated in the centre by a circular aperture, to allow of the insertion of a magnifying eye-piece.
Sir Isaac Newton, in 1669, hit upon the expedient of placing the small mirror at an angle to the large one. He was therefore able to dispense with the perforation of the large mirror, by reflecting the image on to a lens situated in the side of the tube of the telescope. Of course, in all these instruments, the small mirror and its attachments, placed in the tube between the reflector and the heavens, obscured a great deal of the light that would otherwise fall directly on the reflector. How
- ↑ "Les lunettes que construisit Galilée, celles qui lui servirent à decouvrir les satellites de Jupiter, les phases de Vénus et à observer les taches du soleil, grossirent successivement 4, 7, et 32 fois les dimensions Iinéaires des astres. Ce dernier nombre l'illustre astronome de Florence ne le dépassa pas. En remontant autant que j'ai pu faire aux sources où je devais espérer de trouver quelques données précises sur les instruments à I'aide desquels Huygens et J. D. Cassini firent leurs belles observations, je vois que les lunettes de 12 et de 23 pieds de Iong, de 2 1-8 pouces d'ouverture qui conduisirent Huygens à découverte du premier satellite de Saturne et à la détermination de la vraie forme de l'anneau grossissaient 48, 50 et 92 fois; rien ne prouve que ces illustres observateurs aient jamais appliqué à leurs immenses lunettes des grossissements Iinéaires de plus de cent cinquante fois. Enfin une lunette travaillée par Auzont (1664) qui avec la colossale longueur focale de 300 pieds ne grossissait que 600 fois."—Arago.
- ↑ Invented by James Gregory of Aberdeen, 1663.
- ↑ In 1672.
