A VISIT TO AN OBSERVATORY. 105 



less than seventy thousand tons of material had to be re- 

 moved. When the question arose as to the erection of 

 the telescope, the first point to be settled was whether it 

 should be a reflecting telescope or be a refractor, that 

 is, whether it should, like the great telescope of Lord 

 Rosse at Parsonstown in Ireland, consist of a large and 

 brilliant mirror at the end of a long tube, from which the 

 rays of light from the stars were to be reflected ; or 

 whether it should be founded on the more familiar prin- 

 ciple of refraction, in which powerful glass lenses are 

 made use of to concentrate the light and render faint 

 objects visible. There was much to be said on both sides, 

 but finally the trustees, after taking counsel with the 

 wisest astronomers all over the world, decided to erect a 

 great refractor. 



The preparation of the object-glass was the next great 

 work to be accomplished. It was to consist of two pieces, 

 one of crown glass and the other of flint glass, and it 

 was to have a diameter of the unparalleled length of 

 thirty-six inches. The fabrication of the actual pieces of 

 glass on which the opticians could work was a matter of 

 the greatest difficulty, and sorely tried the patience of all 

 concerned ; to obtain the rough discs of glass alone no less 

 than six years were required, and they were only finally 

 adopted after twenty unsuccessful trials. At last, how- 

 ever, when Messrs. Alvan Clark, of Cambridgeport, 

 Massachusetts, did obtain suitable pieces of glass whereon 

 to employ their optical skill, another year of assiduous 

 work was necessary to give to the glasses the exact shapes 

 that would render the vision through them perfect. For 

 the execution of this great object-glass the Lick trustees 



