A Manual Of Photography

PHOTOGRAPHIC REGISTRATION. 155 Mr. Jordan has devised an instrument for numerically regis- tering the intensity of the incident beam, which appears to have some peculiar advantages ; a description of which I shall talte the liberty of transcribing. Figure 19 is an elevation of the instrument: it consists of two copper cylinders supported on a metal frame : the interior one is fixed to the axis and does not revolve, being merely the support of the prepared paper; the exterior cylinder is made to revolve about this once in 19. twenty-four hours by a clock movement. It has a triangular aperture cut down its whole length, as shown in the figure, and it carries the scale of the instrument, which is made to spring closely against the prepared paper. This scale or screen is composed of a sheet of metal foil between two sheets of var- nished paper, and is divided into one hundred parts longitudinally, every other part being cut out, so as to admit the light to the prepared paper without any transparent medium intervening. The lengths of the extreme divisions, measuring round the cylinder, are proportioned to each other as one to one hundred; consequently the lower division will be one hundred times longer passing over its own length than the upper one over its own length^ and the lines of prepared paper upon these divisions will, of course, be exposed to the light for times bearing the same proportion to eacli other. , * Now as the sensitiveness of the paper can readily be adjusted, so that the most intense light will only just tint it through the upper division during its passage under the opening, and ilie most feeble light will produce a similar tint through the lower division during its passage, the number of lines marked on the paper at any given time will furnish a comparative measure of the intensity of solar light at that time, and may be registered as so many degrees of the Heliograph, the name Mr. Jordan