CHAP, 



THE TELESCOPE. 



267 



in length (13 metres), and the mirror had a diameter of 4 feet 10 inches 

 (l'47m). Arago remarked, "Such dimensions are enormous, compared 

 with those of telescopes made up to the present time. Nevertheless, they 

 would appear very insignificant to the people who were told of an 

 imaginary ball given inside the Slough telescope. The originators of 

 this story confounded the astronomer Herschel, with the brewer 

 Meux, a cylinder in which the shortest man could scarcely stand, with 

 the large wooden vats, large as houses, in which beer is made and kept." 



4 



FIG. 203. Sir W. Herschel's large telescope (front view) at the Slough observatory. 



This telescope with its immense weight was, as may be imagined, not 

 easy to move. A very ingenious combination of masts, pulleys 

 and cords, and the continual help of two men, besides an assistant 

 in charge to take the time, were required for working it. More than 

 Ibis, observations with such powerful instruments necessitate a 

 sky of the greatest purity, without which the magnification of the 

 irregularities due to the atmosphere deforms the images and causes 



