150 THE PHOTOGRAPHIC EFFECTS ARK TRANSIENT. 



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. 



598. We see, therefore, that a ray of light which has impinged on the surface of 

 yellow iodide of silver, has lost the quality of causing any farther change on a second 

 similar plate on which it may fall. 



599. In the practice of photogenic drawing, this observation 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 camera is full of light ; 

 its sides reflect back again in all directions 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. 



600. Upon an iodized plate I received a solar spectrum formed by a flint-glass prism, 

 the ray being kept motionless by reflexion from a heliostat, and the plate so arranged as 

 to receive the refracted rays perpendicularly. After five minutes it was mercurialized, 

 and the resulting proof exhibited the place of the more refrangible colours in the most 

 brilliant hues. The lesser refrangible colours had also left their impress of a whitish as- 

 pect, but the region of the yellow was unaltered. All the different rays, therefore, ex- 

 cept the yellow, have the power of changing this particular preparation. Now when 

 a number of pieces of cloth of different colours are placed in the sunbeam, they absorb 

 heat in proportion as their colour is deeper. A black cloth, which does not reflect any 

 of those calorific rays, becomes presently^; and in the same way DAGUERRE'S sensi- 

 tive preparation absorbs all the rays which have any chemical action on it, and reflects 

 the yellow only, which does not affect it. In this particular lies the secret of its vast 

 sensitiveness, compared with the common preparations of the chloride and bromide of 

 silver. 



601. 2d. That as a body warmed by the rays of the sun gradually loses its heat by 

 radiation, or conduction, or contact with other bodies, so likewise, by some unknown pro- 

 cess, photographic effects produced on sensitive surfaces are only transient, and gradually 

 disappear. 



602. After a beam of light has made its impression on the iodide, if the plate be laid 

 aside in the dark before mercurializing, that impression decays away with more or less 

 rapidity ; first the faint lights disappear, then those that are stronger. 



603. Having brought three plates to the same condition of iodization, and received 

 the image of a gas-flame in the camera on each for three minutes, I mercurialized one, 

 A, forthwith ; the second, B, I kept an hour, the third, C, forty-eight hours ; the rela- 

 tive appearance of these three images is represented in Jig. 95. 



604. Those who are in the habit of taking Daguerreotypes know how much they 

 suffer when the process of mercurialization is deferred. To show this effect in the 

 extreme, I took four plates, and having prepared all alike, I exposed half of the surface 

 of each to a bright sky for eight seconds. 



