how or by whom) at some period subsequent to your sketching the
_scarabæus_?"

"Ah, hereupon turns the whole mystery; although the secret, at this
point, I had comparatively little difficulty in solving. My steps were
sure, and could afford but a single result. I reasoned, for example,
thus: When I drew the _scarabæus_, there was no skull apparent on the
parchment. When I had completed the drawing I gave it to you, and
observed you narrowly until you returned it. _You_, therefore, did not
design the skull, and no one else was present to do it. Then it was not
done by human agency. And nevertheless it was done.

"At this stage of my reflections I endeavored to remember, and _did_
remember, with entire distinctness, every incident which occurred about
the period in question. The weather was chilly (O rare and happy
accident!), and a fire was blazing on the hearth. I was heated with
exercise and sat near the table. You, however, had drawn a chair close
to the chimney. Just as I placed the parchment in your hand, and as you
were in the act of inspecting it, Wolf, the Newfoundland, entered, and
leaped upon your shoulders. With your left hand you caressed him and
kept him off, while your right, holding the parchment, was permitted to
fall listlessly between your knees, and in close proximity to the fire.
At one moment I thought the blaze had caught it, and was about to
caution you, but, before I could speak, you had withdrawn it, and were
engaged in its examination. When I considered all these particulars, I
doubted not for a moment that _heat_ had been the agent in bringing to
light, on the parchment, the skull which I saw designed on it. You are
well aware that chemical preparations exist, and have existed time out
of mind, by means of which it is possible to write on either paper or
vellum, so that the characters shall become visible only when subjected