t
nothing attainable by human ingenuity will be found wanting.

Thus provided, Professor Hale has entered upon the task of studying the
sun, and recording from day to day all the changes going on in it,
using specially devised instruments for each purpose in view.
Photography is made use of through almost the entire investigation. A
full description of the work would require an enumeration of technical
details, into which we need not enter at present. Let it, therefore,
suffice to say in a general way that the study of the sun is being
carried on on a scale, and with an energy worthy of the most important
subject that presents itself to the astronomer. Closely associated with
this work is that of Professor Langley and Dr. Abbot, at the
Astro-Physical 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 doub