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THE POPULAR SCIENCE MONTHLY. 



them. Suppose there are eight portraits in the pack, and that under 

 existing circumstances it would require an exposure of eighty seconds 

 to give an exact photographic copy of any one of them. The general 

 principle of proceeding is this, subject in practice to some variation of 

 details, depending on the different brightness of the several portraits: 

 We throw the image of each of the eight portraits in turn upon the 

 same part of the sensitized plate for ten seconds. Thus, portrait No. 1 



Fig. 2. 



is in the front of the pack ; we take the cap off the object-glass of the 

 camera for ten seconds, and afterward replace it. We then remove 

 No. 1 from the pins, and No. 2 appears in the front ; we take off the 

 cap a second time for ten seconds, and again replace it. Next we re- 

 move No. 2, and No. 3 appears in the front, which we treat as its prede- 

 cessors, and so we go on to the last of the pack. The sensitized plate 

 will now have had its total exposure of eighty seconds ; it is then de- 

 veloped, and the print taken from it is the generalized picture of which 

 I speak. It is a composite of eight component portraits. Those of its 

 outlines are sharpest and darkest that are common to the largest num- 

 ber of the components ; the purely individual peculiarities leave little 

 or no visible trace. The latter being necessarily disposed equally on 

 both sides of the average, the outline of the composite is the average 

 of all the components. It is a band, and not a fine line, because the 

 outlines of the components are seldom exactly superimposed. The 

 band will be darkest in its middle whenever the component portraits 

 have the same general type of features, and its breadth or amount of 

 blur will measure the tendency of the components to deviate from the 

 common type. This is so for the very same reason that the shot-marks 

 on a target are more thickly disposed near the bull's-eye than away from 

 it, and in a greater degree as the marksmen are more skillful. All that 

 has been said of the outlines is equally true as regards the shadows ; 

 the result being that the composite represents an averaged figure, 

 whose lineaments have been softly drawn. The eyes come out with 

 appropriate distinctness, owing to the mechanical conditions under 

 which the components were hung. 



A composite portrait represents the picture that would rise before 

 the mind's eye of a man who had the gift of pictorial imagination in an 

 exalted degree. But the imaginative power even of the highest artists 

 is far from precise, and is so apt to be biased by special cases that may 

 have struck their fancies, that no two artists agree in any of their typi- 



