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:
Sensitometer Sensitiveness, expressed in terms
number. of a "Wet Plate."
10 1
11 1-1/3
12 1-3/4
13 2-1/3
14 3
15 4
16 5
17 7
18 9
19 12
20 16
21 21
22 27
23 36
24 48
25 63
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.
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