The Dictionary Of Photography

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

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Exposure
Tylar and Packard's Exposure Meter. This consists of a metal tube, the interior of which is provided with holes and increasing thicknesses of a translucent material which are brought before the holes by means of an external pointer, which indicates the exposure on a tablet affixed to the outside. This is open to the same objection as the last instrument, but differs from it in that it is directed to the subject, and not to the screen.
IVarnerke's Phosphorescent Photometer. The principle of this is the excitation of a patch of luminous paint by the light falling upon the subject for one second, and then the estimation of the brightness of the luminescent paint by means of translucent screens of increasing thickness.
Ballard's Actinometer. This depends for its action on the power of luminous paint to retain and throw out again the light impressions it has received, and it consists of a square tube of wood, having at one end a hinged cover, with, on the inside, a prepared surface and a spot in the-centre. The method of using is to open the cover, and look through the tube directly at the object to be photographed for half a minute; then, closing the tube, a central blue spot will be seen, which will gradually disappear. The number of seconds which it takes to do this forms the basis of the calculation of a series of tables which accompany the actinometer. The fault of these instruments, which depend upon phosphorescent paint, is that the personal equation of the individual eye comes into play, and that, according to Wiedeman (Eder's " Jahrbuch," 1891, p. 588), if Balmain's luminous paint be exposed for one second, the brightness of the light emitted from that paint is not constant, as shown by the following table : -
After the lapse of
and the shorter the exposure the greater the sudden drop - indeed, there is no constant ratio.
The Ilford Exposure Meter. This is practically a circular slide rule, with discs which can be revolved and set to the
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