and there the matter was put to
the proof with entire success; later in Sweden and in Copenhagen.
The patients who were kept under the red light recovered rapidly,
though some of them were unvaccinated children, and bad cases. In no
instance was the most dangerous stage of the disease, the festering
stage, reached; the temperature did not rise again, and they all
came out unscarred.

Finsen pointed out that where other methods of treatment such as
painting the face with iodine or lunar caustic, or covering it with
a mask or with fat, had met with any success in the past, the same
principle was involved of protecting the skin from the light, though
the practitioner did not know it. He was doing the thing they did in
the middle ages, and calling them quacks.

It is strange but true that Dr. Finsen had never seen a smallpox
patient at that time, but he knew the nature of the disease, and
that the sufferer was affected by its eruption first and worst on
the face and hands--that is to say, on the parts of the body exposed
to the light--and he was as sure of his ground as was Leverrier
when, fifty years before, he bade his fellow astronomers look in a
particular spot of the heavens for an unknown planet that disturbed
the movements of Uranus. And they found the one we call Neptune
there.

Presently all the world knew that the first definite step had been
taken toward harnessing in the service of man the strange force in
the sunlight that had been the object of so much speculation and
conjecture. The next step followed naturally. In the published
account of his early experiments Finsen foreshadows it in the words,
"That the beginning has been made with the hurtful effects of this
force is odd enough, since without doubt its beneficial effect is
far greater." His clear head had already asked the question: if the
blue rays of the sun can penetrate deep enough into the skin to
cause injury, why should they not be made to do police duty there,
and catch and kill offending germs--in short, to heal?

Finsen had demonstrated the correctness of the theory that the
chemical rays have power to kill germs. But it happens that these
are the rays that possess the least penetration. How to make them go
deeper was the problem. By an experiment that is, in its simplicity,
wholly characteristic of the man, he demonstrated that the red blood
in the deeper layers of the skin was the obstacle. He placed a piece
of photographic paper behind