A Manual Of Photography

160 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY. THE DYNACTINOMETER. The Dynactinometer is thus described by the inventor :—It consists of a thin metallic disc, perfectly blade, having a slit ex- tending from its centre to the circumference, fixed on an axis revolving through a permanent metallic disc, perfectly white. The white disc has also a slit from its centre of the exact length of the radius of the blade disc; and by means of these two slits, which are so adjusted that the black disc can intersect the white disc, and by revolving, gradually cover the whole white area, the space of the white surface on which the black disc can be superposed forms itself a sort of dial, which is divided into any number of equal segments, all numbered. The inventor has adopted the number of twenty segments for a large circle in- scribed on the dial, and of eight segments for a smaller circle, after the manner of the divisions of the Decimeter, but on the same plane. These eight segments are numbered in geometrical progression, 1, 2, 4, 8, 16, 32, 64. 27. The blade disc may be made to revolve in such a manner that it shall cover a new segment of the large circle during each second, or any other equal fraction of time. By that means the last segment will have received eight times more light than the first, the black disc having moved over the whole in eight seconds. , The differences of photogenic intensities are hardly observable when they follow the arithmetical progression: the instrument is so constructed that it may indicate the intensities in the geometrical progression. The first segment remains alva)s