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78 HISTORY OF COLOR PHOTOGRAPHY
For this reason, great care must be used not to confuse American Scheiner with the European.
In the early days of photography it had been assumed, and most probably verified by the crude experimental procedures that were available at that time, that a reciprocal relationship existed between the intensity of exposure, the time of exposure, and the density of the final image. This relationship could be expressed by the equation
I X T = constant.
I is the intensity of exposure and T is the time of exposure. This may be explained to mean that if the intensity of the exposing light is decreased by 50 per cent, it will be necessary to increase the time of exposure by a like amount, in order to obtain the same result in image intensity. An exposure of five seconds at/:n is identical to an exposure of twenty seconds at/: 22, since the intensity of exposure is four times that which it is at/: 2 2. This is known as the Reciprocity Law. The general validity of this law was first questioned by Abney, and experimentally found to be not true by Schwarzschild. No adequate explanation has yet been made to explain why reduction in exposure intensity followed by an exact mathematically equivalent increase in exposure time, should yield an image density that is sometimes considerably lower than the one made at a higher intensity level. But the truth of this anomaly has been well established.
Consider two emulsions, the first of which has a rating of Weston 40, while the second has a rating of Weston 4. We will suppose that we are making color separations from a still object, such as an oil painting. The object is illuminated in the normal manner, by means of two lights situated so that the lines from their centers to the center of the object make 45-degree angles with the plane of the object. The intensity of illumination is such that it will require an exposure of approximately twenty seconds through the red filter, when using the emulsion with a rating of Weston 40. After exposure and development, it is found that a very flat negative results, indicating that the fast emulsion is not the proper material to use. The slow material will have a much higher contrast, and the apparent flatness can be overcome by a higher development gamma. In accordance with the relative speed ratings of the two emulsions, it would be supposed that an exposure ten times that of the other would be required. If that technique were followed, it would be found that the negative would be considerably overexposed. It will require an exposure only three or four times that given the first material to yield a usable negative.
This can be answered by the statement that the reciprocity law failed. The rating Weston 40 has been assigned to the first emulsion by a speed determination in which the intensity of illumination was such that an exposure of a fraction of a second was required. This will be identical to the conditions under which the emulsion would be used in actual practice. The speed rating