212 ANNUAL OF SCIENTIFIC DISCOVERY. 



of measuring the diameter of the sun to a very minute fraction of a second, 

 by combining photography with the principle of the electric telegraph ; the 

 first being employed to measure the element space, the latter the element 

 time. The result is, that about one twenty-thousandth of a second is suf- 

 ficient exposure to the direct light of the sun to obtain a distinct mark on a 

 sensitive collodion plate, when developed by the usual processes. 



A circular wooden disc, nineteen inches diameter and half-an-inch thick, 

 was mounted on an iron axis, so that it revolved easily by an impulse given 

 by pressing the finger with a jerk on the outer edge. 



About half-an-inch from the rim there was a circular aperture half-an-inch 

 diameter, at the back of which the black paper was pasted. This paper was 

 perforated by a needle, leaving a hole one-sixtieth of an inch diameter. 

 It was found that the utmost velocity that could be given to the disc was five 

 revolutions in a second ; and after four seconds, it was reduced to three revo- 

 lutions per second. At each revolution the space described by the hole was 

 about fifty inches. 



The revolving disc was placed behind the folding doors of a darkened 

 chamber, so that when one wing was opened to the extent of a few inches, 

 the sun's light struck the disc at the lower part of its revolution. Having 

 made the preliminary arrangements, the observation was as follows : 



First, the maximum rotatory motion was given to the disc. A prepared 

 sensitive plate was held close behind the disc (about a quarter of an inch 

 from it), at the part where the sunshine struck. This plate was kept slowly 

 moving in the direction of the radius of the disc. An assistant quickly 

 opened and shut the door, allowing the sunshine to act for about a second. 

 The latent image on the plate being developed, was found to consist of four 

 or five concentric lines. This was repeated several times with different 

 plates. 



Taking the velocity of the aperture to be 1 50 inches per second, which is 

 certainly under the mark, and the breadth of the hole one-sixtieth of an 

 inch, the duration of the sun's full action on any one point must have been 

 about one-nine-thousandth of a second. 



The photographic process employed was as follows : 



" Albumen on glass iodized by tincture of iodine, twenty grains to one 

 ounce of spirit. 



" The silver bath, fifty grains nitrate of silver to one ounce water, and 

 twelve drops nitric acid. 



" The developing solution three parts water to one of acetic ;.cid, and the 

 mixture nearly saturated with protosulphate of iron." 



The above was afterwards tried comparatively with the collodion pro- 

 cess, and found to be considerably inferior in quickness of taking an impres- 

 sion, the ratio being two or three to one. 



ON THE USE OF THE PRISM IN QUALITATIVE ANALYSIS. 



Dr. Gladstone, in a paper read before the London Chemical Society, re- 

 cently, remarked that hitherto the indications of color have played a very 



