Page:Collected Physical Papers.djvu/228



The uncertainty in regard to satisfactory explanation of photographic action is due to the experimental difficulties in studying the problem. As for instance:—

To arrive at a correct idea of the changes produced, it is necessary to measure the minute effects produced by radiation on the extremely thin layer—perhaps only a few molecules deep—of the sensitive substance. In order to ascertain this, it is desirable to begin with the study of some elementary substance in which its effects are attended with few secondary complications. And,