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90 CARVER, TALBOT, AND LOOMIS [J. S. M. P. E.
Spokiness occurs both with 35-mm and with 16-mm film but does not appear to cause -projection difficulties with 35-mm film. With 16-mm film, possibly because there is greater tendency for the spokes to "set" in the film, or possibly because of the lower pressure on the gate shoes in the projectors, in-and-out of focus effects are sometimes produced by the film distortion resulting from spokiness. Spokiness is generally the result of overdrying in processing, resulting in film of high positive curl, and of loose winding, which allows film of high curl to spoke more readily. We must choose, therefore, a middle course between overdrying and underdrying. The former may result in focusing difficulties as a result of spokiness ; the latter may give thermal "in-and-out of focus" effects such as described in the paper, "Effect of High-Intensity Arcs upon 35-mm Film Projection."1
Two distinct kinds of spokiness may be observed. If curly film is wound with the concave side out, the spokes visible on the two sides of the film will always be opposite each other. If the roll is wound with the concave side in, the spokes will be alternate. The spokes seen on one side of the roll will never be opposite those on the other side of the roll.
Embossing. — Embossing was a common defect prior to the advent of the rear type shutter, which came into use at about the same time as did the sound movies. It has always been considered an important defect from the point of view of projection quality. It is obviously due to differential heating of portions of the image due to varying densities throughout the image. The blacker portions get hotter and are shrunken by the heat. Sometimes this effect is most pronounced at the frame lines, when it becomes known as frame-line embossing. During the experiments discussed in the preceding paper,1 considerable heavy embossing was produced by the extreme arc temperatures used, but in no case was the embossing sufficient to cause any focusing difficulties in the film. Measurements of the actual depths of the embossing gave no values higher than 0.0003 inch, which is well within the range of the depth of focus of the projection lenses. Although it may possibly be that under certain circumstances embossing may increase the tendency of film to show in-and-out of focus effects, we have never found a single case of embossing that by itself gave focusing difficulties. One fact was observed, however, that fresh film, and especially film not thoroughly dried, tended to emboss more than well seasoned and dried film. Since it is also a fact that insufficient drying and seasoning tends to