138 ANNUAL OF SCIENTIFIC DISCOVERY. 



in effecting reduction. But, if exposed to light, it continues fora 

 certain time thereafter to retain the vibrations it received ; and 

 just for so long as these vibrations continue, will it be instantly 

 decomposed if brought into contact with any substance which 

 would have caused its decomposition had the two been subjected 

 to the action of light together. 



Iodide of silver, if exposed to light in the presence of pyrogallic 

 acid and nitrate of silver, is reduced. If the iodide be exposed 

 separately, it is thrown into a state precisely similar to that of a 

 phosphorescent body, except that it continues to vibrate in unison 

 with the actinic instead of the illuminating rays ; and so long as 

 this condition remains, if it be brought into contact with the above- 

 mentioned substances, the effect is the same as if they had been 

 exposed together to ordinary light. 



For this property of light I propose the name of actinescence, a 

 name which, though not in every respect suitable, has the great 

 merit of indicating the parellelism of the phenomenon to that of 

 phosphorescence. 



The more we examine these phenomena, the more we shall per- 

 ceive that actinescence must, so to speak, exist. For different 

 phosphorescent bodies emit light of very different colors, showing 

 that their respective capacities of prolonged impression are con- 

 fined to rays of a certain refrangibility differing for each in each 

 case. Now we know that the actinic influence accompanies rays 

 of a certain refrangibility, especially the violet, the indigo, and 

 the rays immediately beyond the visible. The permanence, there- 

 fore, of these actinic rays under suitable circumstances is no more 

 difficult of conception than that of any other rays ; and that this 

 permanence exists for illuminating rays is a fact which has been 

 known and recognized for centuries. 



On what, then, does the faculty of receiving a latent develop- 

 able impression depend ? 



On the possession by the body of two properties : First, that of 

 being decomposed when brought into contact with certain agents 

 in the presence of light. Second, that of being able to retain the 

 influence of the chemical rays, so that on being brought into con- 

 tact with these agents, after removal from the light, the same de- 

 composition may be brought about. 



The first of these properties is sensitiveness to light. 



The second is actinescence. 



The joint possession of the two renders a body capable of re- 

 ceiving a latent or physical image. 



It is easy to conceive that a body may be actinescent without 

 being sensitive to light. In fact, substances that phosphoresce 

 with a blue light are probably actinescent also, but not being sen- 

 sitive to light, they of course can form no latent image. 



To this class undoubtedly belong those substances which pos- 

 sess the property hitherto deemed so mysterious, that of storing 

 up chemical power after exposure to light. When this fact was 

 first published by Niepce de St. Victor it was received almost with 

 ridicule. But in the views here explained, this remarkable fact 



