Observatory of the Smithsonian Institution, who have recently
completed one of the most important works ever carried out on the
light of the sun. They have for years been analyzing those of its
rays which, although entirely invisible to our eyes, are of the
same nature as those of light, and are felt by us as heat. To do
this, Langley invented a sort of artificial eye, which he called a
bolometer, in which the optic nerve is made of an extremely thin
strip of metal, so slight that one can hardly see it, which is
traversed by an electric current. This eye would be so dazzled by
the heat radiated from one's body that, when in use, it must be
protected from all such heat by being enclosed in a case kept at a
constant temperature by being immersed in water. With this eye the
two observers have mapped the heat rays of the sun down to an
extent and with a precision which were before entirely unknown.

The question of possible changes in the sun's radiation, and of
the relation of those changes to human welfare, still eludes our
scrutiny. With all the efforts that have been made, the physicist
of to-day has not yet been able to make anything like an exact
determination of the total amount of heat received from the sun.
The largest measurements are almost double the smallest. This is
partly due to the atmosphere absorbing an unknown and variable
fraction of the sun's rays which pass through it, and partly to
the difficulty of distinguishing the heat radiated by the sun from
that radiated by terrestrial objects.

In one recent instance, a change in the sun's radiation has been
noticed in various parts of the world, and is of especial interest
because there seems to be little doubt as to its origin. In the
latter part of 1902 an extraordinary diminution was found in the