with his work. The "Thumbograph" having been fixed in position, the
light from a powerful incandescent gas lamp, fitted with a parabolic
reflector, was concentrated on it, and the camera racked out to its
proper distance.

"What are those figures intended to show?" inquired Miss Gibson,
indicating the graduation on the side of one of the guides.

"They show the amount of magnification or reduction," Thorndyke
explained. "When the pointer is opposite 0, the photograph is the same
size as the object photographed; when it points to, say, x 4, the
photograph will be four times the width and length of the object, while
if it should point to, say, ÷ 4, the photograph will be one-fourth the
length of the object. It is now, you see, pointing to x 8, so the
photograph will be eight times the diameter of the original thumb-mark."

By this time Polton had brought the camera to an accurate focus and,
when we had all been gratified by a glimpse of the enlarged image on the
focussing screen, we withdrew to a smaller room which was devoted to
bacteriology and microscopical research, while the exposure was made and
the plate developed. Here, after an interval, we were joined by Polton,
who bore with infinite tenderness the dripping negative on which could
be seen the grotesque transparency of a colossal thumb-mark.

This Thorndyke scrutinised eagerly, and having pronounced it
satisfactory, informed Mrs. Hornby that the object of her visit was
attained, and thanked her for the trouble she had taken.

"I am very glad we came," said Miss Gibson to me, as a little later we
walked slowly up Mitre Court in the wake of Mrs. Hornby and Thorndyke;