26o MEMORIES OF MY LIFE
obviously possible to photograph superposed images on a screen by the simultaneous use of two or more lanthorns. What was common to all of the images would then appear vigorous, while individual differences would be too faint for notice. There would, however, be great difficulty in accurately superposing them without the aid of expensive apparatus. Then the idea occurred to me that no lanthorns were needed for the purpose, but that the pictures themselves might be severally adjusted in the same place, and be photographed successively on the same plate, allowing. ,a fractional part of the total time of exposure to each portrait.
My earlier experiments were with the full-face photographs of criminals. I selected three which were not greatly unlike, and were of the same size, as judged by measuring the vertical distance between the pupils of the eyes and the parting of the lips. Out of a thin card I cut a window of the size of the portrait, and fastened two threads over it, one vertical, the other crossways. Lastly I made a pin-hole in the card on either side of the window. Thus provided, I laid each portrait in turn on the table, and adjusted the card until the cross line passed over the pupils of the eyes, and the vertical line bisected the interval. Then I pricked through the two pin-holes the paper on which the portrait was. I could thus hang all three portraits one behind the other on two pins that projected from a board, with the assurance that the principal features of each face would occupy an identical position in front of a fixed camera. I photographed them in turns. The camera was uncapped during one-third of the normal time of exposure while