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392 DUNDON, BROWN, AND CAPSTAFF [j. s. M. p. E.
has been found much simpler and more accurate to develop a standard exposure for a definite time, stop development instantly, and compare the image so obtained with a previously prepared standard. Such a strip can be examined easily by reflected light, either in white light or with a bright safelight. If the time of development for the test strip is so selected that the last available exposure step (shadow detail) just fails to become visible in the fresh developer, then a very great difference will become evident with any less active developer. With some developers, this time may be 30 seconds or less but with borax developer a time of 1 or 2 minutes is best.
Motion picture positive film is advised for the test strips because it can be handled with less danger of light fog than negative film. The test can be made directly in the tank or a sample of the developer can be tested in a graduate or other suitable vessel if care be taken to see that the temperature is correct.
Technic Advised. — A strip of motion picture positive film containing a standard exposure, preferably a print from a uniform density step tablet, is used. The strip of film is quickly dipped into the developer, noting the time accurately to the second. During immersion, it should be agitated gently. Three or four seconds before the required time is up, the strip should be lifted from the developer and so held that it can be plunged into the stop bath without delay when the time is up. In other words, the time of the test is measured from the moment the strip enters the developer to the moment it enters the stop bath. The strip should be agitated when first put into the stop bath. After 10 seconds it can be examined satisfactorily, but if it is to be made permanent for keeping, it should be kept in the stop bath for 2 minutes and then washed for about 10 minutes. In handling the strip, it can merely be dipped by hand with a clip on the lower end for weight, or a special holder can be devised by attaching clips to a monel wire frame. Contamination from fingers or dirty clips should, of course, be avoided. The stop bath can be used most conveniently in a graduate or tube deep enough to permit the entire strip to be suspended in it.
Composition of the Stop Bath. — The formula for the stop bath is as follows:
Potassium iodide 20 grams
Glacial acetic acid 20 cc.
Potassium alum 40 grams
Sodium sulfite 1 gram
Water to 1 liter