OPTICAL INSTRUMENTS. 



367 



The largest telescope ever constructed is that made by Lord Rosso. This 

 instrumeiit, which is a rellecting telescope, is located at Parsonstown, in 

 Ireland. Its external appearance and method of mounting is represented in 

 Fig. 305. The diameter of tlie speculum is 6 feet, and its weight about 4 tons. 

 The tube in which it is placed is of wood hooped with iron, 52 feet in length, 

 and 7 feet in diameter. It is counterpoised in eveiy direction, and moves 

 between two walls, 2-1: feet distant, 72 feet long, and 48 feet high. The ob- 

 server stands on a platform which rises or fails, or at great elevation upoa 

 Bhding galleries which draw out from the walL 



This telescope commands an immense field of vision, and it is said that ob- 

 jects as small as 100 yards' cube, can be distinctly observed by it in the mooa 

 at a distance of 240,000 miles.* 



What is a Magic 727. The Magic Lantern is an optical in- 

 Lantem? strumcnt ada])ted for exhibiting pictures paint- 

 ed on glass in transparent colors, on a large scale, by means 

 of magnifying lenses. 



Fig. 306. 



It consists of a metallic box, or lantern, A A', Fig. 306, containing a lamp, 

 L, behind which is placed a metalhc concave mirror, p q. In front of the 

 lamp are two lenses, fixed in a tube projecting from the side of the lantern, 

 one of which, m, is called the illuminator, and the other the magnifier. The 

 objects to be exhibited are painted on thin plates of glass, which are intro- 

 duced by a narrow opening in the tube, c d, between the two lensea The 

 mirror and the first lens, m, serve to illuminate the painting in a high degree^ 

 for the lamp being placed in their foci, they throw a briUiant light upon i^ 

 and the magnifying lens, n, which can slide in its tube a little backward and 

 forward, is placed in such a position as to throw a highly magnified image if 

 the drawing upon a screen, several feet off, the precise focal distance being 

 adjusted by sliding the lens. The further the lantern is withdrawn from the 



♦ By the aid of this mighty instrument, "one of the most wonderful contributions of 

 art and science the world has yet seen," what astronomers have before called nebula, on 

 account of their cloud-like appearance, have been discovered to be stars, or suns, analo- 

 gous, in all probability, in constitution, to our own sun. In the constellations Andro- 

 meda and the sword-hilt of Orion, both of which are visible to the naked eye, theso 

 cl<>ud-like p&tches have been seen as clusters of stars. 



