The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Ebonite                                                  Effective Aperture
Agitate the chloride of lime with 30 ozs. of water, dissolve the potash in the remainder, mix, and filter.
Ebonite. A modification of indiarubber made by heating it with sulphur under pressure. It is used for making dishes, instantaneous shutters, etc., and, though brittle and affected by heat, answers well, from its great lightness and hardness.
Ebunrneum Process. See Ivorytype.
. Effective Aperture. The effective or working aperture of the lens is that portion which actually comes into use with the various
diaphragms, and the size of the working aperture is dependent not only on the size of the diaphragm, but also on the distance of the object; but for comparison of lenses, and in the ordinary course of testing a lens, parallel rays only are considered in estimating the effective aperture. According to Dr. Stolze, " with any but parallel rays of light, such as those proceeding from near objects under otherwise equal conditions: (i) with single lenses,
with diaphragm in front, the effective aperture increases as the distance of a luminous point decreases ; (2) with doublets, with diaphragm between the lenses, the effective aperture increases as the distance of a luminous point increases." The diagrams given show very clearly how the working aperture of single and doublet lenses is respectively equal to and greater than the diameter of the diaphragm aperture. In both figures f is the