Mark Three: The Adventure of the Marked Plates
Nudge. Find out how the spirit photographer was caught in February 1922: what the investigator did to the plates before the sitting. Edmund did the same thing to his.
Harder
In February 1922 the investigator Harry Price marked the plates in secret before the sitting, so that a plate swapped in would come back without the mark. Edmund's pin made the three specks. A and B have none, and A's clock says it was taken before anyone sat down. C and D are Edmund's own, and only D shows anyone with him.
The mark, in words
Four plates as four buttons in a row: Plate A, Plate B, Plate C and Plate D. Tap one and then Hold it to the light.
The answer
The answer is d.
Price caught the spirit photographer in February 1922 by marking the plates in secret; the plates with spirits on came back without the mark. Edmund marked his with a pinhole triangle. A and B are unmarked and show the same cut-out face of Kit, and A's clock reads ten to six, before the sitting. C and D are genuine. D shows a girl at a fire, in a grate that has been cold since 1912.