History of telescopes

The credit for the invention of the telescope has been a subject of discussion. Thus, because Democritus announced that the Milky Way is composed of vast multitudes of stars, it has been maintained by some that he could only have been led to form such an opinion from actual examination with a telescope. Other passages from the Greek and Latin authors have similarly been cited to prove that the telescope was known to the ancients. But we are no more warranted in drawing such a conclusion without any evidence other than casual remarks, however sagacious, than we should be justified in stating that Seneca was in possession of the theories of Newton because he predicted that comets would one day be found to revolve in periodic orbits.

Reflecting telescopes

Until Newton's discovery of the different refrangibility of light of different colours, it was generally supposed that object-glasses of telescopes were subject to no other errors than those which arose from the spherical figure of their surfaces, and the efforts of opticians were chiefly directed to the construction of lenses of other forms of curvature. James Gregory, in his Optica Promota (1663), discusses the forms of images and objects produced by lenses and mirrors, and shows that when the surfaces of the lenses or mirrors are portions of spheres the images are curves concave towards the objective, but if the curves of the surfaces are conic sections, the spherical aberration is corrected. He was well aware of the failures of all attempts to perfect telescopes by employing lenses of various forms of curvature, and accordingly proposed the form of reflecting telescope which bears his name: the Gregorian telescope. But Gregory, according to his own confession, had no practical skill; he could find no optician capable of realizing his ideas, and after some fruitless attempts was obliged to abandon all hope of bringing his telescope into practical use. Newton was the first to construct a reflecting telescope.

Related Topics:
James Gregory - 1663 - Conic section - Spherical aberration - Gregorian telescope

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When in 1666 he made his discovery of the different refrangibility of light of different colours, he soon perceived that the faults of the refracting telescope were due much more to this cause than to the spherical figure of the lenses. He overhastily concluded from some rough experiments (Optics, bk. i. pt. ii. prop. 3) that all refracting substances diverged the prismatic colours in a constant proportion to their mean refraction; and he drew the natural conclusion that refraction could not be produced without colour, and therefore that no improvement could he expected from the refracting telescope (Treatise on Optics, p. 112). But, having ascertained by experiment that for all colours of light the angle of incidence is equal to the angle of reflexion, he turned his attention to the construction of reflecting telescopes.

Related Topics:
1666 - Angle of incidence - Angle of reflexion

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After much experiment he selected an alloy of tin and copper as the most suitable material for his specula, and he devised means for griding 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 spherical aberration formed the chief faults 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.

Related Topics:
Alloy - Tin - Copper - Satellite - Jupiter

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Encouraged by this success, he made a second telescope of 6k-in. focal length, with a magnifying power of 38 diameters, which he presented to the Royal Society of London in December 1671.

Related Topics:
Royal Society of London - 1671

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A third form of reflecting telescope was devised in 1672 by Cassegrain (Journal des Savants, 1672).

Related Topics:
1672 - Cassegrain

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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) presented to the Royal Society a reflecting telescope of the Newtonian construction, with a metallic speculum of 6-in. aperture and 623/4-in, focal length, having eyepieces magnifying up to 230 diameters.

Related Topics:
1723 - John Hadley - Sextant

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The instrument was examined by Pound and Bradley, the former of whom reported upon it in Phil, Trans., 1723, No. 378, p. 382. After remarking that Newton's telescope had lain neglected these fifty years, they stated that Hadley had sufficiently shown that this noble invention does not consist in bare theory. They compared its performance with that of the object-glass of 123-ft. focal length presented to the Royal Society by Huygens, and found that Hadley's reflector will bear such a charge as to make it magnify the object as many times as the latter with its due charge, and that it represents objects as distinct, though not altogether so clear and bright.

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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 ft.; 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 (Smith's Opticks, bk, iii. ch. I).

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But it was reserved for James Short of Edinburgh to give practical effect to Gregory's original idea.

Related Topics:
James Short - Edinburgh

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Born at Edinburgh in 1710 and originally educated for the church, Short attracted the attention of Maclaurin, professor of mathematics at the university, who permitted him about 1732 to make use of his rooms in the college buildings for experiments in the construction of telescopes. 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. Short then adopted telescope-making as his profession, which he practised first in Edinburgh and afterwards in London. All Short's telescopes were of the Gregorian form, and some of them retain even to the present day their original high polish and sharp definition. Short died in London in 1768, having realized a considerable fortune by the exercise of his profession.

Related Topics:
1710 - Maclaurin - 1732 - 1768

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