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for a given emulsion of a certain density. This is, of course, true as long as the emulsion is observed under the same optical conditions. If these are changed, however, the impression caused by the graininess will change also and — for certain types of emulsions — to an amazingly large degree. If, for instance, an emulsion is projected through an optical system of small aperture, the resulting graininess will appear different from the impression gained when a system of large aperture is employed.
Figs. 4a to 4d illustrate this effect: Two emulsions were chosen, one of large (a and b ) and one of small (c and d ) graininess, however, of approximately the same density. Each emulsion was photographed twice through a microscope at a magnification of 335 diam. and care was taken that exactly the same section of the emulsion was used so that a and b, and c and d are identical each, as far as the arrangement of the grains is concerned. The difference between a and fe, and c and d respectively consists only in the aperture of the illuminating and observing system in which respect a and c, and b and d are identical. The difference between both pairs consists, thus, only in the fact that the aperture for a and c was larger than for b and d approximately 5 x. In spite of the fact that the type of optics, focal length, magnification and emulsion region are identical, it is obvious that the graininess impression received from b and d is considerably larger than that of a and c respectively. The difference in aperture means, in this case, that the images of a and c were produced by light coming from a wide angle, whereas, b and d were produced
from a small angle, i. e. from almost parallel light.
The physical reason for this phenomenon is due to the scattering of light on the grains within the emulsion which results in a decrease of contrast at the border of each grain, resulting at the same time in a decrease of optical definition. The wider the angle under which the emulsion is illuminated and observed, the larger is, of course, the amount of scattered light received by the optical system and vice versa.
In the practical use of photographic emulsions the aperture of the optical system employed varies considerably depending on the particular purpose, e. g., for the use of projecting a microscopic image of the sound track upon the photo-cell in sound reproduction. the aperture is very large, whereas objectives of small aperture are used in general for the projection upon the screen. Hence, for the same emulsion, different values of graininess will have to be used with regard to the final purpose of the emulsion.
The problems connected with graininess, as outlined above very briefly, are by far not the only ones in this field, as for instance the question to what extent the graininess of a negative determines the graininess of a positive made from it, — the question to what extent the graininess of a given emulsion depends on its gamma and the relation between the color of the light by which an emulsion is projected— , play a large role in a field in which, due to lack of an instrument for the objective determination of the graininess, little research has been done in the past. In the next instalment more of these questions shall he discussed in detail.
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