PHOTO PLATES—WET AND DRY.
Reading edition
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 number. Sensitiveness, expressed in terms of a "Wet Plate."
101
111-1/3
121-3/4
132-1/3
143
154
165
177
189
1912
2016
2121
2227
2336
2448
2563
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 developer4-8° C.16-17° C.26-28° C.
Time of development1 min.3° W.8° W.13° W.
"2 min.9½° W.10° W.15° W.
B.—Pyrogallic Developer.
Temperature of developer1-2° C.26-28° C.
Time of development¼ min.6° W.10° W.
"3 min.14° W.15° W.
INTENSIFIER FOR WET PLATES. By MAJOR WATERHOUSE.
The collodion process is still preferred for reproducing black and white designs, drawings, engravings, etc., where very dense negatives are desirable. The fixed and washed plate is put in a bath of bromide of copper (ten per cent. solution); the film whitens immediately, and when the color is even all over, the plate is taken out and plunged into a bath of the ordinary ferrous oxalate developer. It takes a dark olive tint, which is very non-actinic, the shadows meanwhile remaining very clear.—Photo. News.