232 CHEMICAL RAYS AND RADIANT HEAT. [MEMOIR XVII. 



of light depends upon the absorption of those rays by 

 sensitive bodies, etc. 



I iodized a plate to a golden yellow color, and exposed 

 it to the diffused light of day, setting it in such a po- 

 sition that it reflected specularly the light falling upon 

 it from the window to the objective of the camera-obscu- 

 ra, which formed an image upon a second sensitive plate. 

 The rays falling upon the sensitive plate of course ex- 

 erted 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 sensi- 

 tive 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. 



We see, therefore, that a ray of light which has im- 

 pinged on the surface of yellow iodide of silver has lost 

 the power of causing any further change on a second 

 similar plate on which it may fall. 



In the practice of photography this observation is of 

 much importance, especially when lenses having large 

 apertures are used ; the rays converging upon the sensi- 

 tive 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 shad- 

 ows. 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. 



Upon an iodized plate I received a solar spectrum 

 formed by a flint-glass prism, the ray being kept motion- 

 less by reflection from a heliostat, and the plate so ar- 

 ranged as to receive the refracted rays perpendicularly. 

 After five minutes it was mercurialized, and the resulting 



