my
hand fell upon the emblazoned radium flash torch of the black dator.

As I was about to lay the thing aside as of no value in my present
predicament my eyes chanced upon a few strange characters roughly
and freshly scratched upon the soft gold of the case.

Casual curiosity prompted me to decipher them, but what I read
carried no immediate meaning to my mind. There were three sets of
characters, one below another:

3 |--| 50 T
1 |--| 1 X
9 |--| 25 T

For only an instant my curiosity was piqued, and then I replaced
the torch in my pocket-pouch, but my fingers had not unclasped
from it when there rushed to my memory the recollection of the
conversation between Lakor and his companion when the lesser thern
had quoted the words of Thurid and scoffed at them: "And what
think you of the ridiculous matter of the light? Let the light
shine with the intensity of three radium units for fifty tals"--ah,
there was the first line of characters upon the torch's metal
case--3--50 T; "and for one xat let it shine with the intensity
of one radium unit"--there was the second line; "and then for
twenty-five tals with nine units."

The formula was complete; but--what did it mean?

I thought I knew, and, seizing a powerful magnifying glass from the
litter of my pocket-pouch, I applied myself to a careful examination
of the marble immediately about the pinhole in the door. I could
have cried aloud in exultation when my scrutiny disclosed the almost
invisible incrustation of particles of carbonized electrons which
are thrown off by these Martian torches.

It was evident that for countless ages radium torches had been
applied to this pinhole, and for what purpose there could be but
a single answer--the mechanism of the lock was actuated by light
rays; and I, John Carter, Prince of Helium, held the combination
in my hand--scratched by the hand of my enemy upon his own torch
case.

In a cylindrical bracelet of gold about my wrist was my Barsoomian
chronometer--a delicate instrument that records the tals and xats
and zodes of Martian time, presenting them to view beneath a strong
crystal much after the manner of an earthly odometer.

Timing my operations carefully, I held the torch to the small aperture
in the door, regulating the intensity of the light by means of the
thumb-lever upon the side of the case.

For fifty tals I let three units of ligh