A photographic method of detecting changes in a complicated group of objects . uses than the fields in which it has yet been employed that itseems desirable to describe the method and a few of its possibleapplications. II. GENERAL DESCRIPTION OF METHOD, WITHILLUSTRATIONS The method may be described as follows: A negative is madeof the group of objects in which a change is expected. After thechange is supposed to have occurred a second negative is madewith the same camera (or one of the same kind) on a plate of thesame kind and from as nearly as possible the same position asused in making the f
A photographic method of detecting changes in a complicated group of objects . uses than the fields in which it has yet been employed that itseems desirable to describe the method and a few of its possibleapplications. II. GENERAL DESCRIPTION OF METHOD, WITHILLUSTRATIONS The method may be described as follows: A negative is madeof the group of objects in which a change is expected. After thechange is supposed to have occurred a second negative is madewith the same camera (or one of the same kind) on a plate of thesame kind and from as nearly as possible the same position asused in making the first negative. A positive is printed from oneof the negatives, is superposed upon the other negative so as tobring them into register, and the combination is viewed againsta source of light. When the photographs are properly made, 1 Hodgson and Wilsey, Communication No. 42 of Eastman Kodak Co. Research laboratory, J. , 1» Nos. 2 and 3; Harvard College Observatory Circular No. 79, p. 1. 1810670—20 437 438 Scientific Papers of the Bureau of Standards [Vol. 16. Fig. t.—First photograph. This illustrates the initial appearance of the pile of rocks those parts of the combination which correspond to the unchangedportion of the group of objects will appear as a field of practicallyuniform density, while a change in the group will be revealed bya considerable departure from this uniform density. This is illustrated in Figs, i to 7, inclusive. Fig. 1 shows a pileof rocks before the change. Fig. 2 shows the same pile aftermaking a few changes which, it will be noted, are not 3 shows the appearance when the second negative is super-posed upon the positive of the first. Similarly, Figs. 4 to 7, in-clusive, show the application of the method to structural usually makes little difference in the effectiveness of the methodwhether the positive is made from the first or second illustrations will be referred to later. III. FACTORS OF IMPO
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