and on the Discovery of Latent Light. 349 



for the purpose of making trials on the sunlight in lower lati- 

 tudes, and am grieved to see in the reports that have reached 

 this country of the Proceedings of the British Association, 

 certain announcements, received from Professor Bessel*, of 

 phantoms which can be produced on surfaces by mercury va- 

 pour, by the breath, and other means, — as though the thing were 

 new. Years ago, if you look in your own Journal (February 

 1840, p. 84); Sept. 1840, p. 218; Sept. 1841, pp. 198, 199; 

 you will find that 1 had published facts of the kind ; spectral 

 appearances, that could be revived on metals, glass, and other 

 bodies, by the breath, by vapour of camphor, by mercury 

 vapour, &c. The very purpose for which I described them was 

 the striking resemblance of some of them to Daguerreotype 

 images. I have repeatedly shown, that by placing a coin or 

 any other object on iodized .silver, in the dark, the vapour of 

 mercury will bring out a representation of it. And in one of 

 the papers just quoted, the condition under which camera 

 images can be reproduced on a silver plate, even after the 

 plate has been rubbed with rottenstone, is described. 



I have further 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 Kbnigsburg. In 

 your own Journal, more than a year ago, you printed a long 

 paper written by me on this very t opic (September 1841, 

 pp. 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. 



But I will trouble you no further with these private matters, 

 simply hoping that your numerous readers, who feel an in- 

 terest in such things, will turn for themselves to the pages I 

 have quoted. 



The accompanying photographic impression of the solar 

 spectrum, which I will thank you to give to Sir John Her- 

 schel, 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 beyond 

 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 nega- 

 tive rays seem almost as effective in protecting, as the blue 

 rays are in decomposing iodide of silver. 



* We give among the miscellaneous articles of the present Number, 

 page 409, a report from the Athenaeum of what passed at the meeting at 

 Manchester upon this subject. — Ed. 



