A Photographic Method for Recording Lines of Constant Optical Density in a Negative (Isophotes)

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Abstract
A method of preparing gelatin reliefs from photographic negatives is described, using commercially available materials. When these reliefs are viewed in a suitable interferometer, interference fringes are seen which indicate lines of constant thickness in the relief, hence of constant optical density in the original negative. The use of this method for indicating and measuring positions of maximum or minimum optical density is described, and it is shown how the errors caused by a background of variable density may be reduced.

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