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Fine-Grain Developers— Carlton and Crahtree 443
]\Ib. CbiVRK : With regard to grainiiiess, the Bureau of Standards have a resolution factor determined by several dimensions, and I think the Society in dealing with such a subject should use terms adopted by the Bureau of Standards if they want standardization. They have tested the speeds of various emulsions ; they have factors for that. In this way you know what exposure to give any film or plate. The Bureau of Standards have a very comprehensive paper on this subject, and I think the members should get it. It doesn't cost anything.
Mr. Crabtree : I am sorry to have to repeat that we don 't know yet how to express graininess in strict mathematical terminology. Resolution doesn't cover the matter of graininess — it is merely related to graininess. You can measure the average size of the clumps of grains which you see on the screen but a mathematical measurement of the extent of their movement is difficult to obtain. Also you can have a type of graininess in which the clumps are almost invisible but there is a waviness on the screen which is what we call ''secondary graininess." I don't know how this can be expressed mathematically.
Mr. Briefer : Going back to the study of sulfite in the developer and its effect on the size of the grains. Some time ago we had a discussion on this, and this paper admits that there is a solvent action of the sulfite on the silver grains. That is, we are dissolving away part of the silver grain while it is developing. If we do this, we don't have as much silver halide left and you are asking us to accept the proposal that with different types of developers the grain size is smaller for the same density and the same gamma ; that is, two equal densities developed with different developers have different grain sizes, and that is what I would like explained.
Dr. Carlton : With regard to the solvent action of the sulfite, the total amount of silver halide dissolved away is relatively small. If you were to dissolve one-quarter of the silver halide you would decrease the speed of the emulsion so much that it would be of no value for taking pictures.
Your misgivings about the difficulty of securing the necessary density after the sulfite has dissolved away some of the grains would be justified if we were developing to gamma infinity. In practice we are concerned only with negative gammas ranging from 0.7 to 0.9 under which conditions we are only u»ing a fraction of the total densitygiving power of the emulsion.