During the ten years from 1876 to 1886 photography underwent a complete revolution owing to the introduction of the gelatino-bromide plate and as this new phase gave us a great deal to think about, and a great deal of new experience to acquire, our work during these ten years was not wholly confined to what he has previously indicated, but had been directed towards discovering some method of testing the speed of plates, which was workable in the laboratory and independent altogether of the camera.