T H E SIZE-FREQUENCY DISTRIBUTION OF PARTICLES OF SILVER HALIDE I N PHOTOGRAPHIC EMULSIONS AND ITS RELATION TO SENSITOMETRIC CHARACTERISTICS. 11. T H E METHODS OF DETERMINING SIZE-FREQUENCY DISTRIBUTION BY
E. P. WIGHTMAN AND S. E. SHEPPARD
COMMUNICATION NO. 124 FROM THE RESEARCH LABORATORY EASTMAN KODAK COMPANY
OF THE
Introduction I n the first paper of this series1 we discussed the question of a possible relationship between size-frequency distribution of the particles of silver halide in a photographic emulsion and their sensitivity to light action. The most important work in this field which had been done up to that time was considered in detail. We now turn to the question of the methods for determining size-frequency distribution. For convenience they may be classified as follows :
I. MICROSCOPIC PARTICLES A. Fractionation by gravity (including the ordinary elutriation method) or by centrifugal force, in which the sizes of the particles in the fractions are estimated: 1. By consideration of sedimentation laws (including Westgren’s “wedge” method). 2. By optical control of size determinations by a. Counting the particles in a haemacytometer cell. h. Use of the cameral lucida, projection and determination of projective areas by planimetry or other means, or the same method using photomicrography. Method ( b ) may be applied directly to random samples, and in the optical control in general, either transmitted or dark field illumination may be applied. B. Selective filtration methods, Jour. Phys. Chem., 25, 181 (1921); Brit. Jour. Phot., 68, 169 (1921)
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E . P. Wightman and S.E . ShepFard
11. SUBMICROSCOPIC PARTICLES A. Ultra filtration. B . Fractional coagulation. C. Optical grading by counting in fields of varied light intensity and other optical methods. D. Electrical sedimentation, and other sedimentation methods. E. Amplitude of Brownian movement, per se, or electriccontrolled. The methods given apply to the determination of frequency curves only, and not directly to the relation of size of grain t o sensitiveness. For this we may: a. Prepare plates from fractions obtained by one of the above methods. b. Use Svedberg’smethod1 of investigation of residual grains in thinly coated very dilute emulsions, “one grain thick.” The present paper will be devoted largely to an account of the size-frequency distribution methods themselves, including a new sedimentation and grain-counting method. The new counting method is only an approximate one, since to make it a fairly rapid means of obtaining the distribution i t is necessary to limit the separation of the particles to a comparatively small number of fractions, as explained later. In later papers we shall present the results obtained by the application of more accurate, but also more complicated methods. We shall pay slight attention in the present paper to the class of submicroscopic particles, although mention of some work which has been done in this field will be made because of its bearing on the subject in general. Selective filtration methods are rather impractical for the separation of particles larger than ultramicroscopic size and so will be dismissed without further mention. The basis of most of the experimental work which has been T. Svedberg: Zeit wiss. Phot.,
20, 36 (1920).
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done on size-frequency distribution is sedimentation. It is well known that when a uniform mixture of various-sized particles is allowed to settle, the larger particles sediment with a greater velocity than the smaller, due chiefly to the greater frictional resistance of the liquid medium to the smaller particles per unit of weight, their surface per unit weight being greater. The work which has been done in the past has been either of purely scientific nature, or has been applied to such questions as the mechanical analysis of soils and other problems not connected with photography. Incidentally in many of the investigations, Stokes’ Law has played an important r61e, and the question as to its validity has had to be considered from several standpoints. We shall give a brief discussion of the law and its limitations, since it also plays no small part in our own experimental work. Hist orieal As early as 1899, 0. B. Bogghild,l by means of Schone’s elutriation method was able to separate four groups of fractions of the following diameters: >0.5 mm; 0.5 to 0.05 mm; 0.05 to 0.02 mm;
