Telescope

The sharpness of image in Kepler's telescope is very inferior to that of the Galilean instrument, so that when a high magnifying power is required it becomes essential to increase the focal length. G. D. Cassini discovered Saturn's fifth satellite (Rhea) in 1672 with a telescope of 35 ft. and the third and fourth satellites in 1684 with telescopes made by Campani of 1 oo and 136 ft. focal length. Huygens states that he and his brother made object glasses of 17o and 210 ft. focal length, and he presented one of 123 ft. to the Royal Society of London. Adrien Auzout (d. 1691) and others are said to have made telescopes of from 30o to 600 ft. focus, but it does not appear that they were ever able to use them in practical observations. James Bradley, on Dec. 27, 1722, measured the diameter of Venus with a telescope whose object glass had a focal length of feet. In these very long telescopes no tube was employed. They were termed aerial telescopes.

Reflecting Telescopes.

It was not until the middle of the 18th century that these unwieldy instruments were supplanted by the achromatic telescope. Meanwhile the refracting type of tele scope had a rival in the reflecting telescope invented by Sir Isaac Newton. It was in fact Newton who discovered what was the trouble with the refractor, which led to the need for excessive length. It had been supposed that the only imperfection in the image arose from the error known as spherical aberration, and the efforts of opticians were concentrated on devising lenses of suitable forms of curvature to correct this. In 1666 Newton dis covered the different refrangibility of light of different colours, and he soon perceived that the fault of the refracting telescope was that the light of different colours followed different paths; so that if, for example, the telescope was focused sharply for blue light—the green image would be altogether out of focus and blurred. He over-hastily concluded from rough experiments (Optics, bk. i. pt. ii. prop. 3) "that all refracting substances di verge the prismatic colours in a constant proportion to their mean refraction." If this were true no combination of refracting sub stances could bend the path of the light without introducing col our, and therefore no improvement could be expected in the re fracting telescope. He therefore turned his attention to the con struction of reflectors. The form now known as the Gregorian reflector had been proposed by James Gregory in 1663 ; but he had not succeeded in constructing the instrument practically.

Newton, after much experiment, selected an alloy of tin and copper for his specula, and he devised means for grinding and polishing them. He did not attempt the formation of a parabolic figure on account of the probable mechanical difficulties, and he had besides satisfied himself that the chromatic and not the sphe rical aberration formed the chief fault of previous telescopes.

Newton's first telescope so far realized his expectations that he could see with its aid the satellites of Jupiter and the horns of Venus. Encouraged by this success, he made a second telescope of 61 in. focal length, with a magnifying power of 38 diameters, which he presented to the Royal Society in Dec. 1671. A third form of reflecting telescope was devised in 1672 by Cassegrain. No further practical advance appears to have been made in the design or construction of the instrument till the year 1723, when John Hadley (best known as the inventor of the sextant) pre sented to the Royal Society a reflecting telescope of the New tonian construction, with a metallic speculum of 6 in. aperture and 621 in. focal length, having eye-pieces magnifying up to 23o diameters. The instrument was examined by Pound and Bradley, the former of whom reported upon it in Phil. Trans., 1723.

Bradley and Molyneux, having been instructed by Hadley in his methods of polishing specula, succeeded in producing some telescopes of considerable power, one of which had a focal length of 8 f t. ; and, Molyneux having communicated these methods to Scarlet and Hearn, two London opticians, the manufacture of telescopes as a matter of business was commenced by them. How ever, it was reserved for James Short of Edinburgh to give prac tical effect to Gregory's original idea. Born at Edinburgh in 1710 and originally educated for the church, Short attracted the attention of Maclaurin, professor of mathematics at the univer sity, who permitted him about 1732 to make use of his rooms in the college buildings for experiments in the construction of tele scopes. In Short's first telescopes the specula were of glass, as suggested by Gregory, but he afterwards used metallic specula only, and succeeded in giving to them true parabolic and elliptic figures.

Achromatic Telescopes.

The historical sequence of events now brings us to the discovery of the achromatic telescope. The first person who succeeded in making achromatic refracting tele scopes seems to have been Chester Moor Hall, a gentleman of Essex. He argued that the different humours of the human eye so refract rays of light as to produce an image on the retina which is free from colour, and he reasonably concluded that it might be possible to produce a like result by combining lenses composed of different refracting media. After devoting some time to the en quiry he found that by combining lenses formed of different kinds of glass the effect of the unequal refrangibility of light was cor rected, and in 1733 he succeeded in constructing telescopes which exhibited objects free from colour.