390 THE POPULAR SCIENCE MONTHLY. 



to the essential point — the quality of telescopic vision — the testimony 

 of Mr. S. W. Burnham is in the highest degree encouraging. This 

 well-known observer spent two months on the mountain in the autumn 

 of 1879, and concluded, as the result of his experience during that time 

 — with the full concurrence of Professor Newcomb — that " it is the 

 finest observing location in the United States." Out of sixty nights 

 he found forty-two as nearly perfect as nights can well be, seven of 

 medium quality, and only eleven cloudy or foggy ;* his stay, never- 

 theless, embraced the first half of October, by no means considered to 

 belong to the choice part of the season. Nor was his trip barren of 

 discovery. A list of forty-two new double stars gave an earnest of 

 what may be expected from systematic work in such an unrivaled 

 situation. Most of these are objects which never rise high enough in 

 the sky to be examined with any profit through the grosser atmosphere 

 of the plains east of the Rocky Mountains ; some are well-known stars, 

 not before seen clearly enough for the discernment of their composite 

 character ; yet Mr. Burnham used the lesser of two telescopes — a 6-inch 

 and an 18-inch achromatic — with which he had been accustomed to ob- 

 serve at Chicago. 



The largest refracting telescope as yet actually completed has a 

 light-gathering surface twenty-seven inches in diameter. This is the 

 great Vienna equatorial, admirably turned out by Mr. Grubb, of Dub- 

 lin, in 1880, but still awaiting the commencement of its exploring 

 career. It will, however, soon be surpassed by the Pulkowa telescope, 

 ordered more than four years ago on behalf of the Russian Govern- 

 ment from Alvan Clark & Sons, of Cambridgeport, Massachusetts. 

 Still further will it be surpassed by the coming " Lick Refractor." It 

 is safe to predict that the optical championship of the world is, at 

 least for the next few years, secured to this gigantic instrument, the 

 completion of which may be looked for in the immediate future. It 

 will have a clear aperture of three feet. A disk of flint-glass for the 

 object-lens, 38*18 inches across, and one hundred and seventy kilo- 

 grammes in weight, was cast at the establishment of M. Fell, in Paris, 

 early in 1882. Four days were spent and eight tons of coal consumed 

 in the casting of this vast mass of flawless crystal ; it took a calendar 

 month to cool, and cost two thousand pounds sterling. f It may be 

 regarded as the highest triumph so far achieved in the art of optical 

 glass-making. 



A refracting telescope three feet in aperture collects rather more 

 light than a speculum of four feet. % In this quality, then, the Lick 



* "The Observatory," No. 43, p. 613. 



■}■ " Nature," vol. xxv, p. 537. 



X Silvered glass is considerably more reflective than speculum-metal, and Mr. Com- 

 mon's 36-inch mirror can be but slightly inferior in luminous capacity to the Lick object- 

 ive. It is, however, devoted almost exclusively to celestial photography, in which it has 

 done splendid service. The Paris 4-foot mirror bent under its own weight when placed 

 in the tube in 1875, and has not since been remounted. 



