534,

HISTORY OF PHYSICAL ASTROKOAIY.

demanded by the optician proving too great for his slender means, he re-solved, with the true instinct of genius, to rely upon his own personalresources for attaining the object of his wish; and after a course of perse-vering efforts, he at length, in the year 1774, enjoyed the satisfaction ofsurveying the heavens with a 5-feet Newtonian reflector, -which he con-structed with his own hands. This was speedily followed by otherreflectors of 7-feet, 10-feet, and even 20-feet focal length, all of whichwere exquisite specimens of artistic skill.

Armed with such powerful instruments, Herschel now proceeded toexplore the heavens with all the enthusiasm which genius, stimulated bysuccess, is capable of inspiring, lie soon convinced astronomers that hewas no ordinary amateur, who had betaken himself to the observation ofcelestial phenomena, by the high magnifying powers which he employed,amounting to 2000, 3000, and even in some instances to 6500. Thesenumbers far surpassed the magnifying powers hitherto employed in tele-scopic observations. In 1781 his perseverance was rewarded by the dis-covery of the planet Uranus. Soon afterwards, George III . having bestowedupon him a liberal pension, he abandoned his original vocation, and hence-forth devoted himself exclusively to astronomical pursuits. Having re-moved from Bath, he established himself first at Datchet, in the neighbour-hood of Windsor, and subsequently at Slough , where he continued duringthe remainder of his life to prosecute a career of astronomical discovery,which has few parallels in any age or country. The telescopes whichHerschel employed in the early part of his astronomical observations,were all of the Newtonian construction. In 1786, however, he laid asidethe small mirror, and adopted the form of construction which he dis-tinguished by the appellation of the front view. By giving a slight incli-nation to the speculum, so as to throw the image a little to one side of thetube, it was possible to view the latter directly with an eye-glass. Bythis contrivance the light usually absorbed by the small mirror wassaved, and the illumination of the image increased in a correspondingdegree. In such a telescope it is obvious that the observer looks atthe image with his back turned towards the object. Herschel appliedthis form of construction to all the instruments which he subsequentlyemployed in his astronomical observations. His discovery of two satel-lites around Uranus was a result which speedily followed its adoption.It is right to mention that a similar form of construction bad been alreadyproposed by Lemaire, a Frenchman, as early as the year 1732.

In 1789 Herschel surpassed all his former efforts as a practical opti-cian by the completion of a telescope of 40-feet focal length and 4-feetaperture. This gigantic instrument was no sooner turned towards theheavens than it revealed to him the existence of two satellites aroundSaturn which had hitherto escaped the scrutinies of astronomers. It isimpossible, within a moderate compass, to give even a simple enumera-tion of the multitude of brilliant discoveries in celestial physics whichrewarded the labours of this great astronomer. It may he remarked,however, that admirable as were the immediate results of his telescopicobservations, they would have failed to secure to him the exalted placewhich is now universally assigned to him in the history of astronomical dis-covery, if he had not at the same time been endowed with a mind of rareoriginality and power, combined with a strong turn for speculation.

For many years after the invention of the achromatic telescope, the ma-nufacture of flint-glass continued to be confined to Fmgland, which