conditions of light and shade (say with the light coming from the
right side). (2) I reduced their portraits photographically to the
same size, being guided as to scale by the distance between any two
convenient points of reference in the features; for example, by the
vertical distance between two parallel lines, one of which passed
through the middle of the pupils of the eyes and the other between
the lips. (3) I superimposed the portraits like the successive
leaves of a book, so that the features of each portrait lay as
exactly as the case admitted, in front of those of the one behind it,
eye in front of eye and mouth in front of mouth. This I did by
holding them successively to the light and adjusting them, then by
fastening each to the preceding one with a strip of gummed paper
along one of the edges. Thus I obtained a book, each page of which
contained a separate portrait, and all the portraits lay exactly in
front of one another. (4) I fastened the book against the wall in
such a way that I could turn over the pages in succession, leaving
in turn each portrait flat and fully exposed. (5) I focused my
camera on the book fixed it firmly, and put a sensitive plate inside
it. (6) I began photographing, taking one page after the other in
succession without moving the camera, but putting on the cap whilst I
was turning over the pages, so that an image of each of the
portraits in succession was thrown on the same part of the
sensitised plate.

Only a fraction of the exposure required to make a good picture was
allowed to each portrait. Suppose that period was twenty seconds,
and that there were ten portraits, then an exposure of two seconds
would be allowed for each portrait, making twenty seconds in all.
This is the principle of the process, the details of that which I
now use are different and complex. They are fully explained in the