350 Dr. Draper on certain Photographic Impressions. 



The most remarkable part of the phenomenon is, that the 

 same class of rays makes its appearance again beyond the ex- 

 treme lavender ray. Sir 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, 

 Daguerre's preparation, the time of exposure to the sun fif- 

 teen minutes. 



In this impression, six different kinds of action may be di- 

 stinctly traced by the different effects produced on the mer- 

 curial amalgam. These, commencing with the most refran- 

 gible rays, may be enumerated as follows: — 1st, protecting 

 rays ; 2nd, rays that whiten ; 3rd, 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 

 Daguerreotype process by absorbing all the rays coming from 

 natural objects, except the red, orange, yellow, and green, 

 allowing at the same time diffused daylight to act on the 

 plate. 



This constitutes a great improvement in the art of photo- 

 graphy, because 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. Pictures, produced 

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

 in a coloured landscape. 



If it should prove that the sunlight in tropical regions dif- 

 fers intrinsically from ours, it would be a very interesting 

 physical fact. There are strong reasons to believe it is so. 

 The Chevalier Fredrichstal, who travelled in Central America 

 for the Prussian government, found very long exposures in the 

 camera needful to procure impressions of the ruined monu- 

 ments 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 favour to 

 show me on his return, had a very remarkable aspect. More 

 recently, in the same country, other competent travellers have 

 experienced like difficulties, and as I am informed failed to 

 get any impressions whatever. Are these difficulties due to 

 the antagonizing action of the negative rays upon the po- 

 sitive ? 



Yours truly, 



Uuiversity, New York, J. W. DRAPER. 



Sept. 26, 1842. 



