HISTORY OF PHYSICAL ASTROXOHY.

525

perfected in tlieir construction, and would have been eventually appliedwith success to the great purposes of physical science. The merits ofthe original inventor of the telescope cannot affect the reputation ofthe illustrious Italian, who owed his brilliant discoveries in celestialphysics much less to his good fortune in executing a telescope, than to hissagacity as an interpreter of nature.

Kepler first explained the construction of the telescope formed by acombination of two convex lenses, in his treatise on Dioptrics, which waspublished in 1611. Possessing no aptitude for observation, however, hedid not attempt to reduce his ideas to practice. Scheiner appears to havebeen the first person who actually executed a telescope upon the principlesuggested by Kepler. In his Rosa Ursina, which was published in 1030,he states that, thirteen years previously, he made observations in thepresence of the Archduke Maximilian , with a telescope composed of twoconvex lenses. The Galilean telescope, however, still continued to be pre-ferred, on account of the superior distinctness of the image which it formed.

The first person who appears to have discovered the peculiar value ofthe telescope recommended by Kepler, and who applied it to the purposesof astronomical observation, was Gascoigne. This highly-gifted youngman did not fail to perceive that it possessed an immense advantage overthe Galilean telescope, in affording a situation within the tube where anyobject being placed might be viewed as distinctly as a celestial object.This circumstance suggested to his inventive mind the use of telescopicsights in astronomical observations, and the application of the micrometerto the telescope. The micrometrical measures of that astronomer, pre-served by Flamsteed in the first volume of the Historia Celestis, may beconsidered as the earliest collection of facts that have been establishedby the use of the Keplerian telescope. In this respect they possess aninterest analogous to that associated with the early discoveries of Galileo relative to the physical constitution of the celestial bodies.

It was not until about the middle of the seventeenth century, that tele-scopes composed of two convex lenses came to be generally employed inastronomical observations. The use of such telescopes, however, wasthen suggested, not on account of the advantage which Gascoigne foundthem to possess, but because they afforded a much larger field of viewthan could be obtained by means of the Galilean telescope. The firstperson who distinguished himself by his labours in connexion with theKeplerian, or astronomical, telescope was Huyghens . This distinguishedphilosopher attained great excellence in the grinding and polishing oflenses, and succeeded in constructing telescopes far superior to anyhitherto executed. In the year 1655 he discovered a satellite revolvinground Saturn in sixteen days, by the aid of a telescope 12 feet long,and by prosecuting observations of the planet with the same instrument,he was shortly afterwards enabled to establish the real nature of theappendage with which it is furnished.

As the definition of objects was very imperfect in telescopes of thisconstruction, it was found to be indispensable in every instance whereina high magnifying power was required, to increase the focal length of theobject-glass to an immense extent. Huyghens states that he and hisbrother made object-glasses of 170 and 210 feet focal length! Theseglasses were used without tubes. Huyghens devised mechanical combi-nations of great ingenuity, by means of which they could be directed withfacility towards any object in the visible heavens. From the circumstance