Dr. Eder has recently published, in the Correspondenz, the first of a series of articles embodying the results of his more recent work on gelatino bromide; and we now reproduce the substance of the article in a somewhat abstracted form.
The "sensitiveness of a wet" plate continues to be used as a rough and ready standard of comparison; and, notwithstanding the fact that it is physically impossible to exactly compare the sensitiveness of a wet plate with that of a gelatino bromide film, it is convenient to refer to wet plates as some kind of a rough standard.
Experiments have shown that a gelatine plate which gives the number 10 on the Warnerke sensitometer, may be regarded as approximately corresponding to the average wet plate; and setting out from this point, the following table has been constructed:
expressed in terms
of a "Wet Plate."
The nature of the developer used has, of course, some influence on the sensitiveness of the plates; but in the above cases it is assumed that oxalate developer, without any addition, is used; or pyro., to which ammonia is added at intervals of about thirty seconds, so as to produce a slight tendency to fog; the time of development being from three to four minutes. The numbers are supposed to be read after fixation, the plate being held against the sky.
Schumann's statement that a gelatino bromide plate is less sensitive when developed at 30° C. than when developed at 5°, is contested; the more recent investigations of Dr. Eder serving to demonstrate that a developer at a moderate high temperature acts very much more rapidly than when the temperature is low; but when a sufficient time is allowed for each developer to thoroughly penetrate the film, the difference becomes less apparent. Here are examples:
|A. - Oxalate Developer.|
|Temperature of developer||4-8° C.||16-17° C.||26-28° C.|
|Time of development||1 min.||3° W.||8° W.||13° W.|
|"||2 min.||9½° W.||10° W.||15° W.|
|B. - Pyrogallic Developer.|
|Temperature of developer||1-2° C.||26-28° C.|
|Time of development||¼ min.||6° W.||10° W.|
|"||3 min.||14° W.||15° W.|