The two Jannens, father and son, appear to have used their tele scopes in observing the heavens ; and the latter is said to have remarked four email stars near Jupiter : it has been concluded from thence, that be was the first discoverer of the satellites of that planet ; but though this may be, he probably did not continue his observations long enough to enable him to determine their distances from it, or the times of their revolutions.
The use of the telescope, and, probably, even the knowledge of the fact that it had been invented, must have been for many years con fined to the north of Europe ; for it appears that it was not till the year 1609 that Galileo, who then happened to be at Venice, heard from a German a rumour of the discovery which was said to have been suede in Holland. Tho Italian philosopher states, in the `Sidereus Nencius; that he had then no knowledge of the nature of the instntment, and that he requested a friend at Paris to send him some information con cerning it. On being informed, merely, that it was a tube containing glace lenses, hie acquaintance with the nature of the refraction of light enabled him, It in said, to discover that one of the lenses must have been convex and the other connive, and also to determine the distance at which they should be placed from one another In order that the objects seen through them might appear magnified and distinct. Without however supposing that Galileo was hero guided by theoreti cal considerations merely, it is easy to conceive that as lenses of different forms were then in use for spectacles, ho might have obtained from an optician sonic which were of different degrees of contexity and con cavity ; and after a few trials he must have found such as would constitute an instrument possessing magnifying power.
The telescopes which he constructed consisted of one convex object. glass and one concave eye-glass, which were placed at the extremities of a leaden tube ; and the first of them magnified the heights and breadths of objects three times only Soon afterwards ho made one which magnified eight times ; and subsequently ho succeeded in form ing a telescope with a magnifying power which caused objects to appear about thirty times as great as they appear to the unassisted eye.
The knowledge which man had acquired of the visible heavens received many important accessions from the discoveries that Galileo was enabled to make by means of the telescope. Except the sun and moon, not one of the celestial bodies had hitherto been observed to have any visible form or magnitude, and it was to the eye of reason alone that those appeared to be anything but plane surfaces. The
fixed stars and the planets were alike known only as luminous and ill-defined points ; but when seen through a telescope, the planets were found to have certain magnitudes, and some of them to undergo varia tions of form ; while the fixed stars appeared unchanged, or only divested of the radiance with which they seem to be surrounded when seen by the naked eye ; and hence it became obvious that the former must constitute a distinct group of bodies infinitely nearer the earth than the others. The sun, from the spots observed on his surface, was found to revolve on its axis, and consequently was ascertained to be globular ; and the light and dark spaces on the niece] were distinctly perceived to be mountains and valleys, nearly resembling those features on the surface of the earth. Galileo relates, in the work above men tioned, that in the year 1610 he discovered the four satellites of Jupi ter, and observed that they revolved about that planet as our moon revolves about the earth. Nearly at the same time lie observed that Saturn presented a remarkable appearance : at first lie thought it was accompanied by two smaller planets; but on using a telescope of supe rior magnifying power, these were found to be portions of a vast annu lus which surrounds Saturn without touching his surface; and soon afterwards lie ascertained the fact that Venus exhibited phases similar to those of the moon.
The species of telescope which was used by Galileo continued for several years unchanged; yet it is extremely defective, on account of the small extent of the field of view which it affords when its magni fying power is considerable; and the Batavian or Galilean telescope, as it was called, is now chiefly used in the form of an opera-glass. It is due to the memory of Kepler to state that he pointed out (in his Dioptrica ') the possibility of forming telescopes with two lenses, both of which are convex ; but be did not reduce his ideas to practice by the construction of such an instrument, and the honour of having been the first to do so is to be attributed to the Jesuit Scheiuer, who, in his ' Rosa Ursine,' (1650), given a description of telescopes with one convex eye-glass. Ho observes that they cause the images of objects to appear in inverted positions ; and adds, that thirteen years previously he had used such a telescope in the presence of the Archduke Maximilian.