NATIONAL ACADEMY OF SCIENCES. 



diffused daylight even when an ammonia-sulphate of copper cell 

 is interposed. The hands should never rest upon the chest, for 

 the motion of respiration disturbs them so much as to make them 

 of a thick and clumsy appearance, destroying also the representa- 

 tion of the veins on the back, which, if they are held motionless, 

 are copied with surprising beauty. A person dressed in a black 

 coat and open waistcoat of the same color must put on a temporary 

 front of a drab or flesh color, or by the time that his face and the 

 fine shadows of his woolen clothing are evolved his shirt will be 

 solarized and be blue or even black, with a white halo around it. 

 Owing to the circumstance that yellow and yellowish browns re- 

 quire a long time to impress the substance of the daguerreotype 

 persons whose faces are freckled all over give rise to the most ludi- 

 crous results a white portrait mottled with just as many black 

 dots as the sitter has yellow ones." On the 23d of March, 18-10, 

 Dr. Draper presented to the Lyceum of Natural History of New 

 York the first representation of the moon's surface ever taken by 

 photography. The daguerreotype plate was exposed twenty minutes 

 and the image was about an inch in diameter. In this image the 

 places of the dark spots can be indistinctly traced. 



The second method by which he studied the action of the chemi- 

 cal rays was based upon the effect which they produced upon chlorine 

 gas. In 1843 he announced to the British Association that this gas 

 underwent a decided modification under the influence of sunlight, in 

 consequence of the absorption of these chemical rays. In virtue of 

 this change in its character it was now able to unite directly with 

 hydrogen, a property not possessed by chlorine kept in the dark. 

 Hence he announced the discovery of a new imponderable in solar 

 light which was analogous to light and heat, and which was the 

 agent producing chemical change. To this new agent he gave the 

 name " tithonicity," the origin of which he thus describes : " The 

 chemical rays are associated with the rays of light, accompanying 

 them in all their movements, originating with them, and, unless 

 disturbed, continuing to exist along with them. But should a com- 

 pound beam like this fall upon a sensitive surface the chemical rays 

 sink into it, as it were, and lose all their force, and the rays of light 

 are left alone. Photographic results thus obtained from the repos- 

 ing of the chemical rays on the sensitive surface are not, however, 

 in themselves durable, for the rays escape away under some new 

 form. Tithonus was a beautiful youth whom Aurora fell in love 



364 



