£02 Dr. Draper on some Analogies between 



(37.) This corrosion or biting in of the silver plates, by 

 the conjoint action of the mercury and iodine, gives rise to 

 etchings that have an inexpressible charm. Could any plan 

 be hit upon of forcing the iodine to continue its action, the 

 problem of producing engraved Daguerreotypes would be 

 solved. By another process, which will be described here- 

 after, I have succeeded in producing deep etchings from Da- 

 guerreotypes. - 



(38.) I now commence with the proofs of the leading pro- 

 positions set out with in this communication. 



And 1 st, That the chemical action produced by the rays of 

 light, depends upon the absorption of those rays by sensitive 

 bodies, &c. 



Without embarrassing myself here with any considerations 

 of the tints of thin plates, or the colours of natural objects, 

 I shall use the term absorption as expressive of a loss of ra- 

 diant matter, whether that loss arises from a direct union of 

 the luminous molecules with ponderable matter, or is rather 

 a disappearance of effect, caused by the interference of sy- 

 stems of undulations. 



1 iodized a plate to a golden yellow colour, and exposed it 

 to the diffused light of day, setting it in such a position that it 

 reflected specularly, the light falling upon it through the 

 window, to the objective of a camera-obscura, which formed 

 an image of it upon a second sensitive plate. The beams 

 falling upon the sensitive plate, of course exerted their usual 

 influence upon the iodide, which, after the lapse of a short time, 

 began to turn brown. As soon as this effect was observed, 

 I closed the aperture of the camera, and taking out its plate, 

 mercurialized it, but it was found that the rays reflected from 

 the sensitive plate, although they had been converged by a 

 lens four inches in diameter, and formed a very bright image, 

 had lost the quality of changing the iodide of silver. 



(39.) We see, therefore, that a ray of light which has im- 

 pinged on the surface of yellow iodide of silver, has lost the 

 quality of causing any further change on a second similar 

 plate on which it may fall. 



(40.) In the practice of photogenic drawing, this observa- 

 tion is of much importance, especially when lenses having 

 large apertures are used ; the rays which converge upon the 

 sensitive plate are reflected by it in all directions, and the ca- 

 mera is full of light; its sides reflect back again in all direc- 

 tions, on the surface of the plate, these rays, which, if they 

 were effective, must stain the plate in the shadows. But if the 

 plate has been iodized to the proper tint, this light is wholly 

 without action, and hence the proof comes out neat and clean. 



