Telescopes with a single convex eye-glans have been sines designated astronomical, from the circumstance that they were losig employed for celestial observations ; the greater extent of their field of view having caused them, notwithstanding the inversion of the Image, to supersede for that purpose the telescopes of Galileo. It ought to be remarked, however, that telescopes with two eye-glasses, by which the object might be seen in a direct position, as it appears, to the naked eye, were described by Kepler, and constructed by Schreiner; but as they caused the object to appear much distorted and coloured about the margin of the field, they were rapt esteemed. Pere de Rheita, about the same time, constructed for telescopes eye-tubes containing three lenses, which, he observes, afford a better image than those with two. The same person was the inventor of what is called a binocular telescope, that is, an instrument which consists of two telesehpes having equal magnifying powers, and placed near each other in such positions that an object might be oladerved with both eyes st the same time. Attempts have been since made to revive this invention ; but the advantages, if any there be, are more than compensated by the trouble of directing the two tubes to the object. By combining, however, the stereoscopic principle, some advantages are likely to arise, as in the case of the binocular microscope.
The magnifying power of a dioptrical telescope increasing with the ratio which the focal length of the object-glass bears to that of the eye glass, and since, by increasing the focal length of the former without increasing its diameter, the coloured border round the image is dimi nished so that vision is rendered more distinct, the opticians of the 17th century were induced to form, for object-glasses, lenses which were segments of very great spheres—that is, lenses of great focal lengths. Campani, at Bologna, by order of Louis XIV., made tele scopes having object-glaases whose focal lengths were as great as 136 feet ; and with such, Cassini, in 1671, discovered the satellites of Saturn.
Huyghens, who was an ingenious mechanic, as well as a good philosopher, contrived to use an object-glass of long focus for astrono mical purposes without placing the system of lenses in a tube. On the top of a long pole which was planted vertically in the ground, he mounted the object-glass, having fixed it in a frame with joints so that its axis could be moved in any direction by means of a string which was held in the hand of the observer ; and the axis being in a line passing through the celestial body, a short tube containing the eye glasses was fixed to a stand near the ground with its axis in the same direction. An a&ial refractor (as this kind of instrument was =lied), having an object-glasa 123 feet focal length, was made by Hnyghena and presented to the Royal Society ; and with it Dr. Bradley made
some of his astronomical observations. Pound used it to furnish the diameters of Jupiter and Saturn, and the elongations of their satellites, calculated on by Newton in his Principia.' it is described by Huy ghens in his Astroscopia Cotnpendiaria,' which was printed at the liague in 1684. 31. Auzuut is said to have executed an aerial refractor at Paris of 600 feet focal length ; but this proved unmanageable. But the chief merit of lissyghens as an improver of astronomical telescopes consists in his construction of an eye-piece with two lenses so combined as both to enlarge the field of view and diminish the aberrations produced by their spherical forms.
There is some probability that the elder Digges had contrived an instrument which constituted a species of catoptrio or reflecting tele scope ; hut, on account of the obscure manner in which the instrument is described, it will be scarcely necessary to notice further his claim to the honour of the invention. It appears that Pere Mersenne, in his correspondence with Descartes, and In his 'Catoptrics' (1651),auggested the idea of a concave spherical mirror to be used, like the principal lens of a dioptric telescope, for forming in its focus an image of an object ; and that this image being viewed through a convex eye-glass of proper curvature, the original object would appear to be magnified. Descartes, in his reply to Mersenne, which is said to have been written in 1639, makes several objections to the scheme, and no effort was then made to put it in practice. But the great length of the dioptrie tele scopes which were then in use rendering the management of them very inconvenient, ingenious men were induced to attempt a construction in which, with equal magnifying power, much smaller dimensions might be employed. Mr. James Gregory of Edinburgh, in his`Optics Promota' (1663), published a suggestion for forming a telescope by means of the image at the focus of a concave speculum. The mirror was to be of polished metal with a paraboloidal surface, which by the properties of that curve would cause all rays incident upon it in directions parallel to the axis to converge accurately at one point. it is uncertain whether Gregory had any knowledge of blersenne's treatise, or whether the idea originated with himself ; but this is of little consequence, for not being able to find an artist who could exe cute such a speculum, though he came to London for the purpose, the suggestion was abandoned, and men of science continued to direct their inquiries to the means of improving dioptric telescopes.