228 PROCEEDINGS OF THE AMERICAN ACADEMY 



V. Of the Extinction of Phosphorescence by Red Light. 



I turn now to an examination of those parts of the phosphorographic 

 spectrum from which the light has been removed. They are from the 

 line F to the end of the infra-red space, and again for a short distance 

 above the violet. The effect resembles the protecting action in the 

 same region of a photograph. 



Now, if similar effects are to be attributed to similar causes, we 

 should expect to find in the photograph and phosphorograph the 

 manifestation of a common action. 



Several different explanations of the facts have been offered. Her- 

 schel suggested that the photograph might be interpreted on the opti- 

 cal principle of the colors of thin films. Very recently Captain 

 Abney has attributed the appearance of the lower space to oxidation. 

 But this can scarcely be the case in all instances. Mr. Claudet showed, 

 in a very interesting paper on the action of red light, that a daguerreo- 

 type plate can be used again and again by the aid of a red glass, and 

 that the sensitive film undergoes no chemical change. (Phil. Mag., 

 February, 1848). 



It was known to the earliest experimenters on the subject that if 

 the temperatui'e of a phosphorescent surface be raised, the liberation 

 of its light is hastened, and it more quickly relapses into the dark 

 condition. In the memoir to which I have previously referred (Phil. 

 Mag., February, 1851), I examined minutely into this effect of heat, and 

 determined the conditions which regulate it. And since, on the old 

 view of the constitution of the solar spectrum, the heat was supposed 

 to increase toward the red ray, and when flint-glass or rock salt-prisms 

 are employed to give its maximum far beyond that ray, it was sup- 

 posed that this heat expelled the light, and consequently in all those 

 parts of the phosphorus on which it fell the surface became dark 

 through the expulsion or exhaustion of the light. 



I speak of this as " the old view," because, as I have elsewhere 

 shown, the curves supposed to represent heat, light, and actinism so 

 called, have in reality nothing to do with those principles. They are 

 merely dispersion curves having relation to the optical action of the 

 prism and to the character of the surface on which the ray falls. 

 (Pliil. Mag., August, 1872, December, 1872.) 



But this heat explanation of the phosphorescent facts cannot be 

 applied to the photographic. Nothing in the way of hastened or 

 secondary radiation seems to take place in that case. 



In phosphorescence the facts observed in the production of this 



