Those who have not dabbled in photography will require a further explanation of the need of two exposures of the same plate. In the first place, then, I assume that everybody knows that a photograph is the result of light acting on a glass plate coated with a sensitive preparation. When the photographer says, "Now then, a smile, please. Thank you. Steady," you may have noticed that he touches an indiarubber ball. This is a pneumatic release, which momentarily exposes the plate to the light. In that brief period the light eats or rots the sensitive material, at the same time faithfully imprinting upon it an exact image of whatever may be before it.
Now the whole secret of successful photography may be said to depend on this exposure being of the right length. If it is too short, the light has insufficient time to clearly impress the plate with the picture. If it is too long, the image will be too dense.
PROVING A MAN TO BE TRANSPARENT.
So much technique is necessary to a proper explanation of ghost pictures. Now the one second exposure given to our friend the ghost was much too short a time in which to obtain a good portrait of him, but quite long enough to obtain a faint shadowy image, which was just the thing desired.
On obtaining this he quietly walks out of the picture, and we then open the plate for another eight seconds, which is just long enough to obtain a clear and normal photograph of the rest of the picture, including the startled sitter, who has not moved in the meantime. And what is of still greater importance, it gives sufficient time for the foliage and end of the seat to impress themselves through the first picture, and so produce that shadowy transparent texture beloved of all aristocratic varieties of the ghost species.
The tombstone spirit was produced in the same way, a second exposure being given to the tomb, the grass, and the ivy-covered wall behind.
It will be seen that the ghost picture is a particularly simple example of the "fake" photograph.
In the beginning of this article I spoke of the futility of admitting the photograph as evidence.
To prove my assertion, I have been at some pains to prepare a faked photograph that will show how a subject might be "doctored" and put in as conclusive proof.