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tion, which has led other writers into the same mistake.

None of the telescopes, however, made by these artificers were above eighteen inches long, 'nor proper for astronomical observations: but the famous Galileo, astronomer to the Grand Duke of Tuscany, having heard of a sort of optic glass made in Holland which brought objects nearer, set himself to consider how it should be ; and baving ground two pieces of glass into form as well as he could, he fitted them to the two ends of an organ pipe, and by this means made great discoveries in the celestial regions, which he shewed to the Venetian nobility on the tower of St. Mark. From this time Galileo applied himself wholly to improving the telescope; and hence the honour of the invention is frequently ascribed to him, and the instrument has been denominated Galileo's Tube.

All the discoveries made by this sagacious astronomer tended to confirm the truth of the Copernican system, which is now generally received as the true system of the world. In the year 1610, he discovered the four satellites or moons of Jupiter, (as Simon Marius, mathematician to the elector of Brandenburg, did about the same time), but his telescope was not good enough to reach those of Saturn, though he found that planet made an odd appearance, which

very plexed the astronomers to account for, and various hypotheses were formed for that purpose. These all seemed trilling to M. Huygens, who thereupon applied himself to improve the grinding of glasses and perfecting long telescopes, in order to obtain a more accurate knowledge of Saturn and his appendages; and accordingly in 1565, he constructed a telescope of twelve feet, with

much per

which he discovered a stupendous ring encompassing the body of that planet. With a larger tube he discerned a satellite revolving round him, and four more were afterwards discovered by Cassini. -It was Galileo who likewise first observed several dark spots passing over the sun's disk, from whence he appears to have a motion round his axis; and the like phenomena are observable on the surface of some of the planets.

Telescopes are of two sorts, viz. refracting and reflecting. A refracting telescope consists of an object-glass, in the focus of which the image of a distant object is formed, but in an inverted position. This image may be viewed by a single lens or eye-glass, as is usually done in viewing the heavenly bodies, because it is not material whether they appear erect or inverted; but to view terrestrial objects, whose image we would have erect, two other lenses are necessary.

If instead of a convex glass we use a concave one of the same focal length, it will render the object erect, equally magnified, and more distinct and bright: but the disadvantage of this glass is, that it admits of only a small area or field of view, and is therefore not fit to be used when we would see much of an object, or take in a great compass ; it is very useful however in viewing the planets and their satellites, Saturn's ring, &c. And this is properly the Galilean telescope.

But the reflecting telescope is the noblest and most useful of all others, which consists chiefly of specula or mirrors instead of lenses, and is the invention of the great Sir Isaac Newton. put him upon applying his thoughts this way, was the indistinctness of vision by refracting telescopes, which he found was owing to the different refrangibility of the rays of light; from

whence he concluded, that refraction was too unequal a principle, and that lenses, of whatever figures, and how truly soever ground, would never suffice for the perfection of telescopes. Upon this he had recourse to a more equitable principle, viz. that of reflection, and made a telescope consisting of mirrors, the first hint whereof he owns he took from Dr. Gregory's optics. This instrument has since received some farther improvements, insomuch that a reflecting telescope, not more than twenty inches long, may be made to magnify ań object as much as a refracting one sixteen feet in length.

55. The Microscope.

To the invention of the telescope succeeded that of the microscope, an instrument contrived to magnify the smallest objects, so that they may be viewed distinctly. By whom microscopes were invented is not certainly known, but probably wé owe the discovery to the Hollanders, since Huygens tells us that Drebel, a Dutchman, had the first microscope in 1621, and that he was reputed the inventor of it, though the honour is claimed by Fontana, a Neapolitan, who dates it from the same year. They are properly distinguished into single and compound, the former consisting only of one glass, and the latter of several duly combined: and some of these are contrived to do their office by refraction, others by reflection, and others by refraction and reflection conjointly. A single microscope is either a lens or a spherule; but as little glass spheres may be made much smaller than any lens, so the best microscopes, or those which magnify most, are of the spherical kind. Mr. Gray, and others after him, have contrived water-microscopes, consisting of spherules or lenses of water instead of glass; but these magnify less, and are therefore less esteemed. Hollow glass spheres, of the diameter of half a digit, filled with spirit of wine, are frequently used for microscopes, but these magnify still less than the former. With regard to double or compound microscopes, the best are those which consist of one object-glass and two eye-glasses ; for as objects appear dim when viewed through many glasses, part of the rays being reflected in passing through each, the multiplying of lenses is not advisable. The double microscope contrived by our countryman, Mr. Marshall, is of a commodious and curious structure ; but the pocket-microscope of Mr. Martin, furnished with a micrometer to measure the smallest part of an object, is perhaps the plainest, the most expeditious for use, and the cheapest of any of the compound sort hitherto invented.--Here it may be observed, that any telescope is converted into a microscope, by removing the object-glass to a greater distance from the eye-glass : and since the

distance of the image is various, according to the distance of the object from the focus, and it is magnified the more as its distance from the object-glass is greater, the same telescope may be successively converted into microscopes which magnify the object in different degrees.

Many ingenious foreigners, as well as our countrymen, have applied themselves with great diligence to improve the microscope, and by means thereof to lay open the secret wonders of nature : and indeed what surprising discoveries may we not expect in the minute parts of the

creation, from an instrument that makes a small grain of sand appear as big as a nut, or a hair of the head above an inch in diameter ? But of all those who have bent their studies this

way, none have pursued them with more unwearied application, or made more curious and important discoveries than the celebrated M. Leewenhoeck, a Hollander; and therefore as we are treating of that country, it may not be amiss to be a little more particular with regard to that gentleman's microscopes, great part of which he bequeathed to the Royal Society at his decease.

This legacy consists of a small Indian cabinet, in the drawers of which are thirteen little boxes or cases, each containing two microscopes handsomely fitted up in silver ; all which, not only the glasses, but the apparatus for managing them, were made by Mr. Leewenhoeck himself. Each microscope had an object placed before it, and the whole was accompanied with a register of the same in his own hand-writing, as being desirous that the gentlemen of the society should be enabled without trouble to examine several of those objects on which he had made the most considerable discoveries, and which he thought in a particular manner deserved their attention.

Among these objects are some globules of blood; the eye of a gnat; small pipes, which compose the tooth of an elephant; part of the crystalline humour from the eye

of a whale; the instruments from whence a spider spins the threads that compose its web; the animalcules found in various substances ; a bunch of hair from the insect called a hair-worm; the double silk spun by the worm; the eye of a fly; and several other subjects proper for microscopical observations.

It would be endless to enter into a detail of

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