, where the sun
came in. It was warm and pleasant; but was that all? Was it only the
warmth that made the birds break into song when the sun came out on
a cloudy day, made the insects hum joyously and man himself walk
with a more springy step? The housekeeper who "sunned" the
bed-clothes and looked with suspicion on a dark room had something
else in mind; the sun "disinfected" the bedding. Finsen wanted to
know what it was in the sunlight that had this power, and how we
could borrow it and turn it to use.

The men of science had long before analyzed the sunlight. They had
broken it up into the rays of different color that together make
the white light we see. Any boy can do it with a prism, and in the
band or spectrum of red, yellow, green, blue, and violet that then
appears, he has before him the cipher that holds the key to the
secrets of the universe if we but knew how to read it aright; for
the sunlight is the physical source of all life and of all power.
The different colors represent rays with different wave-lengths;
that is, they vibrate with different speed and do different work.
The red vibrate only half as fast as the violet, at the other end of
the spectrum, and, roughly speaking, they are the heat carriers. The
blue and violet are cold by comparison. They are the force carriers.
They have power to cause chemical changes, hence are known as the
chemical or actinic rays. It is these the photographer shuts out of
his dark room, where he intrenches himself behind a ruby-colored
window. The chemical ray cannot pass that; if it did it would spoil
his plate.

This much was known, and it had been suggested more than once that
the "disinfecting" qualities of the sunlight might be due to the
chemical rays killing germs. Finsen, experimenting with earthworms,
earwigs, and butterflies, in a box covered with glass of the
different colors of the spectrum, noted first that the bugs that
naturally burrowed in darkness became uneasy in the blue light. As
fast as they were able, they got out of it and crawled into the red,
where they lay quiet and apparently content. When the glass covers
were changed they wandered about until they found the red light
again. The earwigs were the smartest. They developed an intelligent
grasp of the situation, and soon learned to make straight for the
red room. The butterflies, on the other hand, liked the red light
only to sleep in. It was made clear by many such experiments that
the chemi