in succession, upon the
same sensitised photographic plate. I may add that it is perfectly
easy to superimpose optically two portraits by means of a stereoscope,
and that a person who is used to handle instruments will find a
common double eyeglass fitted with stereoscopic lenses to be almost
as effectual and far handier than the boxes sold in shops."

Mr. Spencer, as he informed me, had actually devised an instrument,
many years ago, for tracing mechanically, longitudinal, transverse,
and horizontal sections of heads on transparent paper, intending to
superimpose them, and to obtain an average result by transmitted
light.

Since my address was published, I have caused trials to be made, and
have found, as a matter of fact, that the photographic process of
which I there spoke enables us to obtain with mechanical precision a
generalised picture; one that represents no man in particular, but
portrays an imaginary figure possessing the average features of any
given group of men. These ideal faces have a surprising air of
reality. Nobody who glanced at one of them for the first time would
doubt its being the likeness of a living person, yet, as I have said,
it is no such thing; it is the portrait of a type and not of an
individual.

I begin by collecting photographs of the persons with whom I propose
to deal. They must be similar in attitude and size, but no exactness
is necessary in either of these respects. Then, by a simple
contrivance, I make two pinholes in each of them, to enable me to
hang them up one in front of the other, like a pack of cards, upon
the same pair of pins, in such a way that the eyes of all the
portraits shall be as nearly as possible superimposed; in which case
the remainder of the features will also be superimposed nearly enough.
These pinholes correspond to what are technically known to printers
as "register marks." They are easily made: A slip of brass or card
has an aperture cut out of its middle, and threads are stretched
from opposite sides, making a cross.[22] Two small holes are drilled
in the plate, one on either side of the aperture. The slip of brass
is laid on the portrait with the aperture over its face. It is turned
about until one of the cross threads cuts the pupils of both the eyes,
and it is further adjusted until the other thread divides the
interval between the pupils in two equal parts. Then it is held
firmly, and a prick is made through each of the holes.

[Footnote 22: