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RADIATION. 1 8 1

solution of iodine only the dark rays are transmitted.
Determining, then, by means of a thermo-electric pile, the
total radiation, and deducting from it the purely obscure,
we obtain the amount of the purely luminous emission.
Experiments, performed in this way, prove that if all the
visible rays of the electric light were converged to a focus
of dazzling brilliancy, its heat would only be one-ninth of
that produced at the unseen focus of the invisible rays.

Exposing his thermometers to the successive colors of
the solar spectrum, Sir William Herschel determined the
heating power of each, and also that of the region beyond
the extreme red. Then drawing a straight line to represent-
the length of the spectrum, he erected, at various points,
perpendiculars to represent the calorific intensity existing
at those points. Uniting the ends of all his perpendiculars,
he obtained a curve which showed'at a glance the manner
in which the heat was distributed in the solar spectrum.
Professor Muller, of Freiburg, with improved instruments,
afterward made similar experiments, and constructed a
more accurate diagram of the same kind. We have now to
examine the distribution of heat in the spectrum of the
electric light; and for this purpose we shall employ a par-
ticular form of the thermo-electric pile, devised by Melloni.
Its face is a rectangle, which by means of movable side-
pieces can be rendered as narrow as desired. We can, for
example, have the face of the pile the tenth, the hundredth,
or even the thousandth of an inch in breadth. By means
of an endless screw, this linear thermo-electric pile may be
moved through the entire spectrum, from the violet to the
red, the amount of heat falling upon the pile at every point
of its march, being declared by a magnetic needle associated
with the pile.

When this instrument is brought up to the violet end of
the spectrum of the electric light, the heat is found to be
insensible. As the pile gradually moves from the violet

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