same luminous intensity as a portion of the
unbroken surface about two and a half minutes from the limb. These
experiments having been made during a spot-minimum when there is reason
to think that absorption is below its average strength, Vogel suggested
their repetition at a time of greater activity. They were extended to
the heat-rays by Edwin B. Frost. Detailed inquiries made at Potsdam in
1892[731] went to show that, were the sun's atmosphere removed, his
thermal power, as regards ourselves, would be increased 1.7 times. They
established, too, the practical uniformity in radiation of all parts of
his disc. A confirmatory result was obtained about the same time by
Wilson and Rambaut, who found that the unveiled sun would be once and a
half times hotter than the actual sun.[732]

Professor Langley, now of Washington, gave to measures of the kind a
refinement previously undreamt of. Reliable determinations of the
"energy" of the individual spectral rays were, for the first time,
rendered possible by his invention of the "bolometer" in 1880.[733] This
exquisitely sensitive instrument affords the means of measuring heat,
not directly, like the thermopile, but in its effects upon the
conduction of electricity. It represents, in the phrase of the inventor,
the finger laid upon the throttle-valve of a steam-engine. A minute
force becomes the modulator of a much greater force, and thus from
imperceptible becomes conspicuous. By locally raising the temperature of
an inconceivably fine strip of platinum serving as the conducting-wire
in a circuit, the flow of electricity is impeded at that point, and the
included galvanometer records a disturbance of the electrical flow.
Amounts of heat were thus detected in less than ten seconds, which,
expended during a thousand years on the melting of a kilogramme of ice,
would leave a part of the work still undone; and further improvements
rendered this marvellous instrument capable of thrilling to changes of
temperature falling short of one ten-millionth of a degree
Centigrade.[734]

The heat contained in the diffraction spectrum is, with equal
dispersions, barely one-tenth of that in the prismatic spectrum. It had,
accordingly, never previously been found possible to measure it in
detail--that is, ray by ray. But it is only from the diffraction, or
normal spectrum that any true idea can be gained as to the real
distribution of energy among the various constituents, visible and
invisibl