LOC's Public Domain Archive
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![Government discovers method to preserve film. (2) Expansion and contraction, like paper and other sheet materials made from cellulose, films expand as they take up moisture and contract as they lose it, and the extent of the change is different in the two directions of them. This may cause some distortion of the image, and therefore is of particular importance where the image must be true to scale, such as in aerial photography. C.O. Pope is shown with a type of expansiveity tester used and which was designed by the Bureau of Standards. Long strips of film are suspended under constant tension in the [cabinet?] in which the humidity is varied by means of [...] solutions. The change in length is indicated [...continuously?] on a scale by means of an optical-level arrangement, 7/8/38](https://cache.getarchive.net/Prod/thumb/cdn4/L3Bob3RvLzIwMTUvMTEvMjUvZ292ZXJubWVudC1kaXNjb3ZlcnMtbWV0aG9kLXRvLXByZXNlcnZlLWZpbG0tMi1leHBhbnNpb24tYW5kLWNvbnRyYWN0aW9uLWxpa2UtMTAyNC5qcGc%3D/40/32/webp "Government discovers method to preserve film. (2) Expansion and contraction, like paper and other sheet materials made from cellulose, films expand as they take up moisture and contract as they lose it, and the extent of the change is different in the two directions of them. This may cause some distortion of the image, and therefore is of particular importance where the image must be true to scale, such as in aerial photography. C.O. Pope is shown with a type of expansiveity tester used and which was designed by the Bureau of Standards. Long strips of film are suspended under constant tension in the [cabinet?] in which the humidity is varied by means of [...] solutions. The change in length is indicated [...continuously?] on a scale by means of an optical-level arrangement, 7/8/38")