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telescope. The eye pieces of our varying magnifying powers take the 

 place of our own eyes, which they resemble, except in power ; they are 

 microscopes. The motion of Venus, giving to her the phases of our moon, 

 can almost be seen by the naked eye. When Gallileo first saw that, he 

 felt the conclusion of her and our earth's revolution around the sun. It 

 established the truth of the Copernican system. He saw with it, for the 

 first time, the satellites of Jupiter, and their revolutions around the planet — 

 another parallel demonstration of the revolution of the planets around 

 the sun. Huygens went further ; he discovered the brightest of Saturn's 

 moons. Titan, He adopted the negative eye-piece, except when spider 

 lines are used for measurement. Many persons ask, how large does the 

 moon look to you ? One says, " as big as a hogshead;" one, " a dinner 

 plate ;" another, " a saucer." Hold up your little finger, with your arm 

 straight, and it covers the moon at full. Photography is now employed to 

 take the faces of planets. Here is one of the moon. Our maps of the moon 

 are far superior to the photograph. Jupiter's belt and Saturn's rings, 

 too — the latter you know when its edge presents itself to our view — is but 

 a luminous fibre stretched across the face of Saturn — a mere spider's web. 



Gregory's reflecting telescope has two concave mirrors, the large one at 

 the lower end, with a small one above — the rays passing through a hole in 

 the centre of the large one from the smaller one, to the eye, give two re- 

 flections. The parabolic curve is the true surface for an object glass or a 

 mirror. 



Mr. Meigs. — This conic section is, therefore, not only the true path of a 

 projectile, but the best pathway of light for our eyes. 



The curvature of the small mirror should be elliptic, but is commonly 

 made spherical ; and then the curve of the large mirror should be the hyper- 

 bola to counteract the aberration of the small one. Herschel's large mir- 

 ror was 4 feet in diameter by three inches thick. This was not invented by 

 Messier. Lord Rosse's reflector is 6 feet diameter and 6 inches thick ; 45 

 per cent of the light is absorbed by the mirror. Silver has been proposed 

 for mirrors, but it readily turns dark, as you see on all your plate. Prof. 

 Steinhiel has electrotyped silver on a mirror ; this forms a beautiful surface, 

 and can of course be renewed as often as you please. The most diaphanous 

 substance reflects some 12 per cent of light, l-12tli of every incident ray. 

 Flint glass disperses light more than crown glass. Refracting telescopes 

 are preferable to reflectors. Great expectations grew, as to celestial disco- 

 veries, from Lord Rosse's, but they have not been met. Rutherford's 

 refractor, of 11 inches aperture, shows Saturn's seventh satellite mirror. 

 An object glass of large size might, perhaps, be sawed ofi" from a mass in 

 shallow crucible, say 2 to 3 feet ; but with all our care, one part of the 

 melted mass will diff"er from the other. Dolland's 4 feet tube, and 4 inches 

 aperture, is a very good telescope. Watson, of Ann Arbor Observatory, 

 is a very good observer ; he determined the true path of the recent comet. 



Mr. Meigs. — Can reflectors be maintained by artificial means at the tem- 



