490 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



the figure of the surface, but only increases the refleting power from two 

 or three per cent, up to more than ninety. This silver coatiug is trans- 

 parent, and shows bright objects, such as the sun, of a light blue tint by 

 transmitted light. 



POWER OF MU. draper's TELESCOPE. 



As regards the degree of excellence that can be reached by such tele- 

 scopes, I can only say that mine can show every object that other instru- 

 ments of similar size do, and more too. I can see the fifth component of 

 Debili.-^sima, which is the minimum visibile in Lasscll's two-foot speculum, 

 and the 18th magnitude pair near 6 Capricorn, discovered by Herschel's 

 eighteen and a-half inch speculum. These demonstrate the light-collecting 

 power of such a silver surface. In tests for sharpness of definition it will 

 separate the blue component of Andromeda with a power of four hundred, 

 and the instrument, on a favorable n^ght, will bear three times that power. 



It must not be supposed, however, that so excellent a result was obtained 

 without laboi*. I have.ground and polished more than a hundred mirrors, 

 of sizes varying from nineteen inches to one-quarter of an inch in diameter. 

 This has involved the construction of several distinct polishing machines; 

 the earlier framed on foreign models, the latter original. Some of the 

 large mirrors have also been polished and corrected for spherical observa- 

 tion by hand. 



THE MOUNTING. 



The mirror is sustained in a walnut tube hooped with brass and sup- 

 ported in a frame which holds the tube at both ends. This is to avoid the 

 tremulous motions so commnn in large instruments. The eye-piece, or, 

 what is the same tiling, the place of the photographic plate, is stationary 

 at all altitudes, and an observer has never to strain himself by awkward 

 positions, but always looks straight forward. AVhen photographs of the 

 moon are taken, the telescope is not driven by clockwork, but is allowed 

 to come to rest completely. The sensitive-plate alone is moved in a direc- 

 tion and at a rate to correspond with the moon's motion. The difference is, 

 that instead of having to carry more than half a ton, the clock has only 

 one ounce to move. Of course, there is no comparison between the pre- 

 cision of movement possible in the two cases. 



THE CLOCK. 



This is a clepsydra of [lecnliar consti notion, and has no wheel work of 

 any kind about it. It is free, tlu>r<'fore, from all irregularities produced by 

 teeth, and also from those caused l)y the oscillating movements of a pen- 

 dulum. 



THE ((BSERVATORY 



at Hastings, Westchester comity, is a building twenty feet square and 

 twenty-two feet high, and is one-lialf exf^uvated out of the solid rock, so as 

 to keep the reflector at a uniform low temperature, and at the same time 

 give steadiness and immohility to the telescope. It stands on a hill two 

 hundred and fifty feet above tiie leviil of the sea, and, in consequence, 

 escapes much of tlu; fog which accumulates in the valleys around, and fre- 

 quently, in the cold part of the night, fills them nearly full. 



The dome which covers the building is sixteen feet in diameter, and is 



