ving placed
the little lamp close to the microscope mirror, adjusted the
spectroscope. Evidently my friend was fixing the position of the "D"
line (or sodium line) in the spectrum.

Having completed the adjustments, he now examined afresh the
blood-smears and the thumb-print, both by transmitted and reflected
light, and I observed him hurriedly draw one or two diagrams in his
notebook. Then he replaced the spectroscope and lamp in the case and
brought forth the micrometer--a slip of rather thin glass about three
inches by one and a half--which he laid over the thumb-print in the
place of the upper plate of glass.

Having secured it in position by the clips, he moved it about, comparing
its appearance with that of the lines on the large photograph, which he
held in his hand. After a considerable amount of adjustment and
readjustment, he appeared to be satisfied, for he remarked to me--

"I think I have got the lines in the same position as they are on our
print, so, with Inspector Johnson's assistance, we will take a
photograph which we can examine at our leisure."

He extracted the camera--a quarter-plate instrument--from its case and
opened it. Then, having swung the microscope on its stand into a
horizontal position, he produced from the camera case a slab of mahogany
with three brass feet, on which he placed the camera, and which brought
the latter to a level with the eye-piece of the microscope.

The front of the camera was fitted with a short sleeve of thin black
leather, and into this the eye-piece end of the microscope was now
passed, the sleeve being secured round the barrel of the microscope by a
stout indiarubber band, thus producing a completely light-tight
connection.

Everything was now ready for taking the photograph. The light from the
window having been concentrated on the thumb-print by means of a
condenser, Thorndyke proceeded to focus the image on the ground-glass
screen with extreme care and then, slipping a small leather cap over the
objective, introduced the dark slide and drew out the shutter.

"I will ask you to sit down and remain quite still while I make the
exposure," he said to me and the inspector. "A very little vibration is
enough to destroy the sharpness of the image."

We seated ourselves accordingly, and Thorndyke then removed the cap,
standing motionless, watch in hand, while he exposed the first plate.

"We may as well take a second, in case this should not turn out quite
p