PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION 489 



ber of our most intelligent photographers, and manj gentlemen interested 

 in the recent beautiful applications of this novel art, listened to the sub- 

 joined paper by Dr. Henry Draper, upon his arrangements for the photo- 

 graphic delineation of celestial objects. The successful achievements of 

 this gentleman in his chosen department of investigation, have become 

 known to many persons in our community, and much interest has been felt 

 in them. To such the description will prove interesting. 



Mr. Draper is a son of the distinguished Dr. John W. Draper, professor 

 of chemistry in the New York University, and is associated with his father 

 as professor of natural science in that institution. 



Dr. Draper said, in the autumn of 1858, I determined to make the largest 

 reflecting telescope in America. Its construction, together with the various 

 improvements successively added, has occupied me up to the present time 

 — more than five years. The instrument, which is nearly sixteen inches in 

 aperture and thirteen feet in focal length, was intended to be devoted to 

 celestial photography, and consequently contains many novelties especially 

 fitting it for that purpose. A description of it was read at the Oxford 

 meeting of the British association in 1860. It has since then been com- 

 pleted, and has now the largest silver reflector of any instrument in the 

 world except that in the Imperial Observatory at Paris. The Smithsonian 

 Institute is preparing to publish shortly a full account of it, which will 

 contain the entire detail of construction. 



THE REFLECTOR. 



The reflecting telescope is greatly superior to the achromatic for photo- 

 graphic purposes. In my instrument a movement of the sensitive plate, 

 one hundredth of an inch on either side of the true focus, visibly injures 

 the image. In the great achromatic at Cambridge, on the contrary, the 

 position of the plate may be varied over an inch without any noticeable 

 change. TWe difference is simply that, while by reflection the visual and 

 chemical rays both converge to the same focus, by refraction they do not. 

 A sensitive plate, put where the eye sees the image sharply, produces a 

 fine result in a reflecting telescope, but- does not in an achromatic. Be- 

 sides this, more light is reflected by a large silver mirror than an achro- 

 matic of equal size can transmit. 



At first I used speculum metal for my mirrors, but abandoned it at Sir 

 John Ilerschel's suggestion ia favor of silvered glass, the reflecting power 

 of the latter being ninety-three per cent., while that of the former is at 

 the best but seventy-five per cent — a large achromatic only transmitting 

 about seventy-five per cent. The glass mirror, too, weighs not more 

 than one-eighth as much as the metal one — the one weighing sixteen 

 pounds, the other one hundred and twenty-eight pounds. It is also greatly 

 more permanent, for if the silver coating which covers the glass concave 

 should by chance be injured, it can be dissolved off easily with nitric acid, 

 and the mirror re-silvered in an afternoon, and this may be repeated indefi- 

 nitely. A person making such a silvered glass reflector is content to take 

 the greatest pains to produce a glass concave of the utmost perfection, for 

 once that is obtained it need never be lost. The thin sheet of silver 

 deposited upon it, only one two hundred thousandth of an inch thick, copies 

 with the last degree of accuvacj the glass beneath, and does not modify 



