I am indebted for the woodcuts to the Editor of
_Nature_, in which journal this memoir first appeared.]

[Illustration: ]

The portraits being thus arranged, a photographic camera is directed
upon 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 variations of detail, 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 sensitised
plate for ten seconds. Thus, portrait No. 1 is in the front of the
pack; we take the cap off the object glass of the camera for ten
seconds, and afterwards 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 remove No. 2,
and No. 3 appears in the front, which we treat as its predecessors,
and so we go on to the last of the pack. The sensitised plate will
now have had its total exposure of eighty seconds; it is then
developed, and the print taken from it is the generalised 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 number 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 skilful. 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 are hung.

[Illus