204 Miscellanies. 



Photography. — We copy the following from the same letter of Prof. 

 Draper from which the foregoing is taken : — 



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 beyond the ex- 

 treme red, which totally and perfectly arrest the action of the light of the 

 sky. This impression was obtained when the thermometer was 96° Fah. 

 in the shade, and the negative rays seem almost as effective in protecting, 

 as the blue rays are in decomposing iodide of silver. 



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

 of rays makes its appearance again beyond the extreme lavender ray. 

 Cir J. 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 silver, Da- 

 guerre's preparation, the time of exposure to the sun fifteen minutes. 



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 fol- 

 lows : — 1st, protecting rays; 2d, rays that whiten ; 3d, rays that blacken ; 

 4th, rays that whiten intensely; 5th, rays that whiten very feebly ; 6th, 

 protecting rays. 



It is obvious we could obtain negative photographs by the Daguerre- 

 otype process by absorbing all the rays coming from natural objects, ex- 

 cept the red, orange, yellow, and green, allowing at the same time diffus- 

 ed daylight to act on the plate. 



This constitutes a great improvement in the art of photography, be- 

 cause it permits its application in a negative way to landscapes. In the 

 original French plan the most luminous rays are those that have least 

 effect, whilst the sombre blue and violet rays produce all the action. Pic- 

 tures, produced in that way, never can imitate the order of light and 

 shadow in a colored landscape. 



If it should prove that the sunlight in tropical regions differs intrinsic- 

 ally from ours, it would be a very interesting physical fact. There are 

 strong reasons to believe it is so. The Chevalier Fredrichstal, who trav- 

 elled in Central America for the Prussian government, found very long 

 exposures in the camera needful to procure impressions of the ruined 

 monuments of the deserted cities existing there. This was not due to 

 any defect in his lens ; it was a French achromatic, and I tried it in this 

 city with him before his departure. The proofs which he obtained, and 

 which he did me the favor to show me on his return, had a very remarka- 



