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photographic deposit which we call graininess. The term "graininess" is used to designate the effect of inhomogeneity exhibited by the photographic deposit owing to the presence of groups or clumps of silver particles. It is the clumping of the grains which limits the size to which a picture can be enlarged before the graininess becomes apparent.
Up to the time that this work was undertaken, no method for the measurement and numerical specification of the graininess of a photographic deposit was available, it being customary to obtain qualitative evaluations by submitting a given picture to inspection by qualified judges. Such procedure obviously is not entirely satisfactory, since determinations made in such a way are subject to wide variation owing to personal opinion, nor is any means provided for recording the results quantitatively. In order to reduce the measurement of graininess to a scientific basis, it was necessary to develop a logical method and to build an instrument for the measurement of this factor. Space will not permit of a complete treatment of the theory of the subject and for a more detailed discussion the reader is referred to other publications (The Measurement of Graininess in Photographic Deposits, by Loyd A. Jones and Noel Deisch, Jour. Frank. Inst., November, 1920, 3. 657).
Fig. 1. Diagrammatic Sketch of Graininess Measuring Apparatus
The instrument constructed for this purpose consists briefly of a microscope system which projects a magnified image of the photographic deposit on a smooth white screen. The screen is observed in a mirror which operates along a track permitting the length of the optical path from the eye of the observer to the screen to be varied at will. In making a determination of graininess, the mirror is driven out until the granular structure of the screen image is no longer visible. The assumption is then made that the graininess of the photographic material is proportional to the distance from the eye of the observer to the screen for this position of the mirror, provided that all the factors controlling the ability of the eye to recognize fine detail remain constant. A diagrammatic sketch of the apparatus is shown in Fig. 1. The photographic material is placed on the microscope substage at 0 and at magnified image is projected onto a screen of magnesium carbonate (Sc) by means of the microscope system and pointolite lamp (Si). M is a mirror which is mounted on a carriage
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