Composite photography consists in the fusion of a certain number of individual portraits into a single one. This is effected by making the objects which are to be photographed pass in succession before the photographic apparatus, giving each of them a fraction of the long exposure, equal to such exposure expressed in seconds and divided by the number of the objects which are to be photographed. Composite photography is interesting when applied to photographs of persons. Theoretically this is what occurs: Features peculiar to each of the portraits, not having been sufficiently exposed, do not take; and the features common to all, having been given a proper exposure, alone leave a visible trace, along the sensitized plate. Therefore, the result obtained may be considered as the type of the race or the family, but, of course, is only of limited value. Our [engraving] shows twelve portraits, six men and six women, some of whom are quite young and some middle-aged, as may be readily seen. An exposure was made in succession of No. 1 to No. 12, that is to say, beginning with the youngest woman and ending with the oldest man; and then from No. 12 to No. 1, that is to say, in inverse order. A man and a woman were interposed, and the experiment was renewed, preserving the same arrangement, but changing the order of the subjects. The result remained constantly the same, as may be readily seen by glancing at the four composites, A, B, C, and D, of the [engraving]. Upon one side the type of six men (composite E) was made, and on the other, of six women (composite F). Here the change produced is very perceptible. It is always the same head; but while before we had a being of indeterminate sex, we find here, with perfect distinctness, a man on one side and a woman on the other. The experimenter wished to see whether twelve other persons (six men and six women), taken from the same population, would give a type analogous to the first. As may be seen (composite G), there is a slight difference, but the character of the head is the same, the difference existing especially in the physiognomy. The same remark may be made as to the composite H obtained from the six women of the preceding group joined with the six women figured Nos. 1, 2, 3, 4, 5, 6, which alone gave the composite F. This observation proves (what was to be foreseen) that the more the number of subjects for each experiment is increased, the greater will be the probability of obtaining the true type of the population studied. On the contrary, when but three are taken, a great risk will be run of generalizing too much. In this case, moreover, each exposure is necessarily too long, since it consists of a third of the normal exposure and is no longer the resultant of the three heads, but their superposition. Hence the slightest increase in the length of one of the three exposures assumes considerable importance.

COMPOSITE PHOTOGRAPHS.

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CHAPTER II.
CHRONOPHOTOGRAPHY.

Instantaneous photography has been of the greatest possible use to science, especially that branch of it which has been termed “chronophotography.” It is to the investigations of Mr. Muybridge and M. Marey that we are indebted for the most valuable researches on the subject. Chronophotography consists in taking a number of photographs of any object at short and regular intervals of time. This is accomplished in many ways, and results obtained are useful for many purposes. The graphic method has been of great service in almost every branch of science, and laborious statistics obtained by computation have been replaced by diagrams in which the variation of a curve expresses in the most striking manner the various phases of some patiently observed phenomena. Furthermore, by the methods of modern science, a recording apparatus has been devised which, working automatically, traces the curves of such physical or physiological events which, by reason of their slowness, feebleness, or their speed, would otherwise be inaccessible to observation. The development of these methods of analyzing movement by photography have enabled the researches of physiological laboratories to become of the greatest possible value. The matter in this chapter is very largely an abstract of M. Marey’s researches, which were originally published in “La Nature,” and their publication in the “Scientific American Supplement” extended over a period of several years. Subsequent to this publication M. Marey wrote a book called “Le Mouvement,” which has been translated by Mr. Eric Pritchard under the title of “Movement.” It is published in the International Scientific Series; and for a more extensive and scientific treatment of the subject than we are able to give here, we refer our readers to this excellent work. M. Marey describes the rudiments of chronography by supposing we take a strip of paper which is made to travel by clockwork at a uniform rate. A pen affixed above the paper marks, as it rises and falls alternately, the various periods and intervals. When the pen comes in contact with the paper it leaves a record in the form of dashes of different lengths at varying intervals. If the dashes should be equidistant it shows that the periods of contact follow one another at equal intervals of time. Now, as it is known that the speed at which the paper travels is so many inches or feet per second, it is an easy matter to obtain an accurate measurement of the duration of contact and of the intervals between. In brief, this is the principle of chronography. Chronophotography is simply an amplification of this system and has many advantages, rendering measurements possible where the moving body is inaccessible. In other words, there need be no material limit between the visible point and the sensitized plate.