JOHN WILLIAM DRAPER. 425 



In 1837 Professor Draper began to publish his researches on the 

 light of the sun, and on the solar spectrum, — a subject to which he 

 often returned down to the year 1873. The discovery of Daguerre 

 turned his attention to photography and photo-chemistry. He antici- 

 pated even Daguerre in the art of taking portraits by the action of 

 light. But before the introduction of collodion a long exposure was 

 necessary. As early as 1840 he obtained a photograph of the moon, 

 about one inch in diameter, after an exposure of twenty minutes. In 

 1842 he announced the paradox of latent light ^ pi'oducing images 

 invisible to the eye until revealed by chemical action upon them ; 

 whereby a new territory was annexed to the solar spectrum at the 

 violet end, corresponding to the calorific extension at the red end. 

 He was successful in photographing the fixed lines in the solar spec- 

 trum, outside even of its visible limits, whether formed by the disper- 

 sion of a prism or by the interference of a fine grating. His paper, in 

 which he proved that the decomposition of carbonic acid by the leaves 

 of plants was produced under the influence of the yellow rays rather 

 than of the violet rays of the sun, which appeared originally in the 

 Proceedings of the American Philosophical Society for 1843, was 

 republished in London, Paris, and Berlin. 



The results of Professor Draper's experiments on the relation of light 

 to heat were given to the public in 1847. He proved that all solid 

 bodies became incandescent at the same temperature, red hot at 977° 

 Fahrenheit; and that the more refrangible rays were successively added 

 at increasing temperatures, and the original rays became at the same 

 time more intense. Melloni, who may be said to have created the 

 science of radiant heat (so widely did he extend its area), was imme- 

 diately attracted to these investigations of Dr. Draper, and testified to 

 the ingenuity and success with which they had been conducted. When 

 Kirchhoff, in 18G2, published an appendix to his researches on the 

 solar spectrum, in which he gives a mathematical foundation for ex- 

 perimental deductions already known, he said : " Draper has derived 

 from experiment the conclusion that all solid bodies begin to glow at 

 the same temperature. But he has observed in his experiments that 

 certain bodies, as chalk, marble, and fluor-spar, shine at a lower tem- 

 perature than they should according to this law : he calls this light 

 phosphorescent, and observes that it is distinguished from the glow by 

 its color. But whatever name may be given to the light, it contra- 

 dicts the law, and a body which shows it cannot satisfy the assumption 

 which is made in proving the law ; it cannot remain unchanged, the 

 temperature remaining the same ; the phosphorescence is not the 



