PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 491 



supported on a point at its centre, instead of on rollers or cannon-balls 

 around the edge, as is usual. It is, of course, moved with much more ease, 

 and, so far from requiring complicated wheelwork, can be turned around 

 by a gentle pressure with the hand. 



The entire management of the telescope is conducted from an observing- 

 chair which follows the telescope. The instrument can be directed to an 

 object, the dome shifted, and the observer himself moved to any desired 

 part of the building by means within immediate reach, and which only 

 require slight exertion, 



THE PHOTOGRAPHIC LABORATOKY 



is attached to the observatory on the western side, only a few feet inter- 

 vening 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. Whenever 

 an inch of rain falls I can collect the tank full. The total amount that can 

 be secured in a year is about forty tons. 



THE PHOTOGRAPHS. 



The instrument has been in working order for eighteen months, but a 

 large part of the time has been unused because of my absence with the 

 Twelfth regiment in Virginia, and on account of the duties of the Natural 

 Science Professorship in the University. 



With my father's (Prof. J. W. Draper) assistance, I have, however, taken 

 some very fine photographs during the past summer. 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 ofiensive 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. It is the largest that has ever been made. 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 repre- 

 sents the moon on a scale of seventy miles to the inch. In the picture 

 before you, attention should be directed particularly to the Apenine range, 

 Copernicus with his reflecting streams, the great groove from Tycho, the 

 numerous craters with an internal cone, the irregularities visible in the 

 bottom of the Mare Imbrium. 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 photographic 

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

 took the audience quite by surprise. It was a view of the moon when 



