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April, 1935] SOUND RECORDING 289
the discussion of these is beyond the scope of this paper. I need only state that within certain limits, which limits may be regarded as very satisfactory for most practical purposes, the necessary conditions can be fulfilled. This paper is concerned with the conditions which are necessary for the operation of stages V, VI, VII, and VIII; that is, with the properties of the negative and positive photographic films and with the conditions under which they must be used in order to ensure satisfactory reproduction of the sound.
Now, the positive film which carries the sound-track from which reproduction is achieved also carries the pictures showing the movement with which the sound is synchronized; and in order that the pictures shall be of satisfactory quality, it is necessary that certain conditions shall be achieved.1 The cycle of operations necessary to produce a photographic reproduction is depicted in Fig. 3, which I owe to Dr. L. A. Jones. In the top right-hand corner will be seen a black cross illuminated by the sun, which will serve as a symbol for any photographic subject. This is reproduced in the left-hand top corner as an image on film for projection. The accuracy with which the picture duplicates in the brightnesses of its tones the different parts of the original subject corresponds to what is generally called the objective phase of tone reproduction.
Let us consider the steps of the photographic process by which the object is translated into the print. The first step is the projection of the object by means of a lens upon the sensitive material, where it forms an optical image and, after the duration of a given time, produces a latent image. This is developed, the material is fixed, washed, and dried, and we obtain a negative. This negative is printed either by contact or projection upon the positive material, producing first an optical image, then a latent image, and then, upon development, a positive print.
When the image falling upon the sensitive material is transformed into a negative, the accuracy of the tone reproduction depends upon the shape of the well-known curve shown in Fig. 4, which represents the growth of density hi a photographic material as the exposure is increased. This shows what is known as the characteristic curve of an emulsion. There are three fairly well defined regions of the curve: Thus, from A to B we have the initial part, convex to the log E axis, which may be termed the region of underexposure*', between B and C, known as the region of correct exposure, the increase of density is practically constant for each increase of exposure; and in the third