414 TRANSACTIONS OF THE 



orbits. By its revelations thousands of nebulge have already been cata- 

 logued, and tens of thousands are still to be. 



The tube of this wonderful telescope is 56 feet long, made of wood one 

 inch thick, and hooped with iron ; its diameter is 7 feet. It is placed be- 

 tween two walls, and commands a view for half an hour on each side of the 

 meridian ; that is, its whole motion, from east to west, is limited tu fifteen 

 degrees. It has a reflecting surface of 4,071 square inches, while that of 

 Herschel's forty-foot telescope had only 1,811. 



Sir James South made a trial of this instrument in 1845, and said, 

 " Never before in my life did I see such glorious sidereal pictures. The 

 nebulae in the Caues Vevatici was resolved into a large globular cluster of 

 stars, not unlike that in Hercules." 



The great nebulas of Orion has been examined with every telescope 

 made since the first inventor, without the remotest aspect of a stellar con- 

 stillation. 



But in March, 1846, Lord Eosse reported that he plainly saw through 

 his telescope, a mass of stars about the trapezium, and that the rest of the 

 nebulae abounded with stars. It has proved that the bright stars, com- 

 prized between the first and fourth magnitudes, are one quarter physically 

 double ; that is to say, one out of four. The probability is, that the num- 

 ber of compound systems is less than the number of insulated stars. Such 

 is admitted to be the power of Rosse's telescope, that if a star of the first 

 magnitude were removed to such a distance that its light would require 

 three millions of years to reach us, this marvelous instrument would show 

 it to our eye. The range opened to us by Kosse's telescope may be appre- 

 hended by the measurement, not of distances in leagues, or tens of millions 

 of leagues or diametei's of our Httle earth's orbit, but of the advancement 

 of light in free space. It requires one hundred and thirty-five years for 

 the flight of light from a fixed star of the sixth magnitude to reach us. 



Leon Foucault formed a telescope speculum of silvered glass, in which 

 experiment he used glass that was not transparent, but in fact imperfect, 

 but still free from air bubbles. On this he deposited a film of silver of 

 uniform thickness, and found it became transparent, transmitting a blue 

 light, and the reflecting surface was exceedingly brilliant. Dr. Greer often, 

 examined, and looked through the difl"erent mirrors constructed on this 

 novel principle, and actually put them to a severe test, by comparing one 

 of seven Inches with an achromatic of fine quality, and pronounced Mr. 

 Foucault's superior. 



I have so frequently used the term achromatic, and may so often use it 

 again before bringing this subject to a close, that I will in this connection 

 say a few words concerning it. 



Achromatism, means the destruction of primary colors, which invariably 

 accompany the image of any object seen through a lens or prism. Light 

 is compounded of rays unequally refrangible and is not homogeneous. In 

 passing into a telescope, some of the rays are refracted and bent out of 



