104 TKANSACTIONS OF THE [FEB. 21^ 



will be made up of all the rays — there will be no such thing as 

 chromatic aberration. Thus if a mirror could be made and 

 would continue of the true parabolic shape, the great and des- 

 perate difficulty in the way of improving the telescopes might be 

 removed. 



In the time of Newton, the reflecting telescopes, however, did 

 not excel the long refracting telescopes. Even after Dolland' 

 discovery, the great difficulty met with in obtaining pure glass 

 made the earlier short achromatic telescopes not much better 

 than the long instruments. But in the latter part of the 18th 

 century a genius arose who solved the problem of the construc- 

 tion of large mirrors. History tells us that '' William Herschel, 

 in 1766, was a church organist and teacher of music, of high 

 repute in Bath. He spent what little leisure he had in the 

 study of mathematics, astronomy and optics. By accident a 

 Gregorian reflector two feet long fell into his hands, and turn- 

 ing it to the heavens, he was so enraptured with the views pre- 

 sented to him that he sent to London to see if he could not 

 purchase one of greater power. The price named was far above 

 his means. He resolved, then, to make one for himself. After 

 many experiments with metallic alloys, to learn which would 

 reflect most light, and many efforts to find the best way of pol- 

 ishing his mirror, and giving it a parabolic form, he produced a 

 five-foot long (Newtonian) reflector, which revealed to him a 

 number of interesting celestial phenomena, though, of course, 

 nothing that was not already known." 



He determined, then, to make the largest telescope that could 

 be made, and after many failures, he produced a telescope hav- 

 ing a mirror two feet in diameter and 20 feet long. At this 

 time, 1781, he discovered the planet Uranus. His fame coming 

 to the ears of the king, George III., that monarch gave him a 

 pension of £220, that he might devote his life to the study of 

 the heavens. He now accomplished his greatest work by mak- 

 ing a reflector four feet in diameter and forty feet long. With 

 this he discovered two new moons of Saturn. 



It was not until 1842 that another great step was taken in the 

 direction of increasing the power of the reflecting telescopes. 

 Then the Earl of Rosse, of Parsonstown, Ireland, constructed 



