118 A2TNUAL OP SCIENTIFIC DISCOVERY. 



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

 avoid the tremulous motions so common in large instruments. The eye- 

 piece, or, what is the same thing, the place of the photographic plate, 

 is stationary at all altitudes, and an observer has never to strain him- 

 self by awkward positions, but always looks straight forward. When 

 photographs of the moon are taken, the telescope is not driven by clock- 

 work, but is allowed to come to rest completely. The sensitive-plate 

 alone is moved in a direction 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 precision of movement possible in 

 the two cases. 



" The observatory where the above-described instrument is erected, 

 is at Hastings, Westchester County, N. Y., and is a building twenty 

 feet square and twenty-two feet high, and is one-half excavated out of 

 the solid rock, so as to keep the reflector at a uniform low temperature, 

 and at the same time give steadiness and immobility to the telescope. 

 It stands on a hill two hundred and fifty feet above the level of the 

 sea. The photographic laboratory is attached to the observatory on 

 the western side, only a few feet intervening between the telescope 

 and developing sink. It contains all the requisite conveniences for 

 taking photographs up to three feet in diameter, and is furnished with 

 a tank which holds a ton of rain-water. This supply is procured from 

 the roof of the buildings, which are on this account painted with a 

 stone paint so as not to contaminate the water. 



" This instrument," said Dr. Draper, " has been in working order 

 for eighteen months, but a large part of the time has been unused be- 

 cause of my absence with the Twelfth Regiment in Virginia, and on ac- 

 count of professional duties. I have, however, taken some fine photo- 

 graphs during the past summer. Some changes have been made in 

 the photographic processes commonly used, in order to fit the pictures 

 for bearing high magnifying powers. I have negatives which can be 

 enlarged by a power of thirty-two without showing granulation or other 

 defects to an offensive degree. The photograph which I show you to- 

 night is nearly two feet in diameter, and is magnified to two hundred 

 and ten times the size of the moon, as seen by the naked eye. I have 

 now another, however, still larger in my observatory, nearly three 

 feet in diameter, made under a power of three hundred and twenty. 

 It represents the moon on a scale of seventy miles to the inch. In the 

 picture before you, attention should be directed particularly to the Ap- 

 ennine Range, Copernicus, with his reflecting streams, the great groove 

 from Tycho, the numerous craters, with an internal cone, the irregu- 

 larities visible in the bottom of the Mare Imbriuna. But it is useless 

 to particularize; there is an almost inexhaustible supply of objects for 

 study and admiration. 



" The Society will see that, although celestial photography may be, as 

 yet, only in its infancy, it is rapidly advancing. Every day is giving 

 origin to improvements, and even now the limit of size in these pictures 

 is rather owing to the great expense and difficulty of working such 

 enormous plates than to any intrinsic defect of the images to be copied." 



Upon the conclusion of his paper, Dr. Draper exhibited a photo- 

 graphic view of the moon's surface, the singular distinctness and beauty 



