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Feb., 1930] CHARACTERISTICS OF SOUND FlLM 203
edge, being largest at the edge and becoming practically constant, that is, no contraction, 5 mm. inside the edge.
In practice these two effects occur simultaneously. The actual change in the image size depends both on the size of the optical image and on the density of the photographic image. Thus for a very small optical image, that is, of the order of magnitude of the one used in obtaining the data represented by Fig. 14, the growth factor is large, whereas the contraction factor is almost negligible. For an image 10 mm. in diameter, however, the reverse would be true, except in the case of extreme over exposures. The combined effect of the two factors is shown in Fig. 16. This image was formed by a slit 0.99 mm. wide placed in contact with the emulsion. The family of curves represents 2, 4, 8, and 16 minutes' development, respectively, from left to right. It is seen from these curves that for low exposures the contraction factor predominates, whereas for higher exposures the growth factor becomes predominant. Just how important these factors are is a subject for further investigations.
DISCUSSION
MR. KEU/OGG: I should like to ask whether sharpness and resolution go together.
DR. SANDVIK: Sharpness and resolution do not necessarily go together. In general, an emulsion which has high resolution also has high sharpness, but they depend on different factors. Sharpness depends on the density gradient at the edge of an image measured in the shadow. The slope of the straight line of the curve is the sharpness, whereas resolving power depends on the slope, and shape of shoulder and toe.