THE SOLAR SPECTRUM ON A DAGUERREOTYPE PLATE. 357 



ran be reproduced on a silver plate, even after the plate has been rubbed with rotten- 

 stone, is described 



643. I have farther seen (Literary Gazette, July 23, 1842, Paris letter), that the fact 

 that light becomes latent in bodies, after the manner of heat, was announced in France 

 as a new and important discovery of Professor MOSER of Konigsburg. In your own 

 Journal, more than a year ago, you printed a long paper written by me on this very 

 topic (September, 1841, p. 196, 204, 205, 206), not merely announcing the fact, but 

 giving rude estimates of the amounts : more exact numerical determinations I have 

 now nearly ready for the press. 



644. But I will trouble you no farther with these private matters, simply hoping 

 that your numerous readers, who feel an interest in such things, will turn for them- 

 selves to the pages I have quoted. 



645. The accompanying photographic impression of the solar spectrum, which I 

 will thank you to give to Sir JOHN HERSCHEL, was obtained in the south of Virginia : 

 probably you can make nothing like it in England: the sunlight here, in New- York, 

 wholly fails to give any such result It proves that, under a brilliant sun, there is a 

 class of rays commencing precisely at the termination of the blue, and extending be- 

 yond the extreme red, which totally and perfectly arrest the action of the light of the 

 sky. This impression was obtained when the thermometer was 96 Fahr. in the shade, 

 and the negative rays seem almost as effective in protecting as the blue rays are in de- 

 composing iodide of silver. 



646. The most remarkable part of the phenomenon is, that the same class of rays 

 makes its appearance again beyond the extreme lavender ray. Sir JOHN HERSCHEL has 

 already stated, in the case of bromide of silver, that these negative rays exist low down 

 in the spectrum. This specimen, however, proves that they exist at both ends, and 

 do not at all depend on the refrangibility. It was obtained with yellow iodide of sil- 

 ver, DAGUERRE'S preparation, the time of exposure to the sun fifteen minutes. 



647. In this impression, six different kinds of action may be distinctly traced by the 

 different effects produced on the mercurial amalgam. These, commencing with the most 

 refrangible rays, may be enumerated as follows : 1st, protecting rays ; 2d, rays that whi- 

 ten ; 3d, rays that blacken ; 4th, rays that whiten intensely ; 5th, rays that whiten very 

 feebly; 6th, protecting rays. 



64 S. It is obvious we could obtain negative photographs by the Daguerreotype pro- 

 cess, by absorbing all the rays coming from natural objects, except the red, orange, yel- 

 low, and green, allowing, at the same time, diffused daylight to act on the plate. 



649. This constitutes a great improvement in the art of photography, because it per- 

 mits its application, in a negative way, to landscapes. In the original French plan, 

 the most luminous rays are those that have least effect, while the sombre blue and violet 

 rays produce all the action. Pictures produced in that way never can imitate the or- 

 der of light and shadow in a coloured landscape. 



650. If it should prove that the sunlight in tropical regions differs intrinsically from 

 ours, it would be a very interesting physical fact There are strong reasons to believe 

 it is so. The Chevalier FREDERICHSTAL, who travelled in Central America for the Pros- 



