154 THE ACTIVE RAYS ARE ABSORBED, 



we restore also its sensitiveness. Here, then, in this remarkable result, we again perceive 

 a corroboration of our first proposition. 



627. I may, however, observe in passing, that although I am describing these ac- 

 tions as if there was an actual absorption of the rays, and that films on metallic plates 

 exhibit colours, not through any mechanism like interference, but simply because they 

 have the power of absorbing this or that ray, there is no difficulty in translating these 

 observations into the language of that hypothesis. When the diffracted fringes given 

 by a hair or wire in a cone of diverging light are received on these plates, correspond- 

 ing marks are obtained, a dark stripe occupying the place of a yellow fringe, and a 

 white that of a blue. I found, more than four years ago, that this held in the case of 

 bromide of silver paper (446), and have since verified in a more exact way with this 

 French preparation. Similar phenomena of interference may be exhibited with the 

 chloride of silver. 



628. We have it, therefore, in our power to exalt or depress the sensitiveness of any 

 compound by changing its optical conditions. Until now, it has been supposed that 

 the amount of change taking place in different bodies by the action of the rays of 

 light, depended wholly on their chemical constitution, and hence comparisons have 

 been instituted as to the relative sensitiveness of the chlorides, bromides, oxides, and 

 iodides of silver, &c. But it seems this liability to change depends also on other prin- 

 ciples, which being liable to variation, the sensitiveness of a given body varies with 

 them. Thus this very iodide of silver, when in a thin yellow film, is decomposed by 

 the feeblest rays of a taper, and even moonlight acts with energy ; yet simply by alter- 

 ing the thickness of its film, it becomes sluggish, blackening in the sunlight tardily, and 

 recovers its sensitiveness again on recovering its yellow hue. 



629. We have now no difficulty in understanding how, in the preparation of ordi- 

 nary sensitive paper, great variations ensue by modifying the process slightly, and how, 

 even on a sheet which is apparently washed uniformly over, large blotches appear 

 which are either inordinately sensitive or not sensitive at all. If, without altering the 

 chemical composition of a film on metallic silver, or even its mode of aggregation, such 

 striking changes result by difference of thickness, how much more may we expect that 

 the great changes in molecular condition, which apparently trivial causes must bring 

 about on sensitive paper, should elevate or depress its capability of being acted on by 

 light ! If I mistake not, it is upon these principles that an explanation is to be given 

 of the successful modes of preparation which Mr. TALBOT and Mr. HUNT have descri- 

 bed, and the action of the mordants of Sir JOHN HERSCHEL. 



630. I therefore infer, 



6th. That the SENSITIVENESS of any given preparation depends on its chemical na- 

 ture and its optical qualities conjointly, and that it is possible to exalt or diminish the 

 sensitiveness of a given compound by changing its optical relations. 



631. 7th. That, as when radiant heat falls on the surface of an opaque body, the 

 rays reflected are complementary in number to those that are absorbed, so in the case of 

 a sensitive preparation, the rays reflected are complementary in number to those that are 

 absorbed. 



