NATURAL PHILOSOPHY. 117 



ment of science. He stated, that in the autumn of 1858, he " deter- 

 mined to make the largest reflecting telescope in America. Its con- 

 struction, 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 pur- 

 pose. It has now the largest silvered-reflector of any instrument in the 

 world except that in the Observatory at Paris." " The reflecting tele- 

 scope," said Dr. Draper, " is greatly superior to the achromatic for 

 photographic purposes. In my instrument, a movement of the sensi- 

 tive 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 with- 

 out any noticeable change. The difference is simply that, while by 

 reflection the visual and chemical rays both converge to the same fo- 

 cus, 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. Besides this, more light is reflected by 

 a large silver mirror than an achromatic of equal size can transmit. 



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

 Sir John Herschell's suggestion in favor of silvered glass, the reflect- 

 ing 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 transmits 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 oft" easily with nitric acid, and the mirror re-silvered in an af- 

 ternoon ; and this may be repeated indefinitely. 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 when once it 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 de- 

 gree of accuracy the glass beneath, and does not modify the figure of 

 the surface, but only increases the reflecting power from two or three 

 per cent, up to more than ninety. This silver coating is transparent, 

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

 mitted light. 



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

 telescopes, I can only say that mine can show every object that other 

 instruments of similar size do, and more too. I can see the 18th mag- 

 nitude pair near (3 Capricorni, discovered by Herschell's eighteen and 

 a half inch speculum ; and in tests for sharpness of definition, it will sep- 

 arate the blue component of Andromeda with a power of four hun- 

 dred, and the instrument, on a favorable night, will bear three times 

 that power. It must not be supposed, however, that so excellent a re- 

 sult was obtained without labor. I have ground and polished more 

 than a hundred mirrors, of sizes varying from nineteen inches to one- 

 quarter of an inch in diameter. 



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



