260 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 super- 

 posing 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 them- 

 selves 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 verti- 

 cal, the other crossways. Lastly I made a pin-hole 

 in the card on either side of the window. Thus pro- 

 vided, 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 photo- 

 graphed them in turns. The camera was uncapped 

 during one-third of the normal time of exposure while 



