sue it beyond the limits of its immediate usefulness. But here,
though the first hint leading to remarkable discoveries was a direct
consequence of his astronomical work, the novelty and interest of the
phenomena observed induced him to follow the investigation very far
beyond the mere solution of the practical question in which it
originated.

Having tried many varieties of shade-glasses between the eye-piece of
his telescope and the eye, in order to reduce the inordinate degree of
heat and light transmitted by the instrument when directed towards the
sun, he observed that certain combinations of colored glasses permitted
very little light to pass, but transmitted so much heat that they could
not be used; while, on the other hand, different combinations and
differently colored glasses would stop nearly all the heat, but allow an
inconveniently great amount of light to pass. At the same time he
noticed, in the various experiments, that the images of the sun were of
different colors. This suggested the question as to whether there was
not a different heating power proper to each color of the spectrum. On
comparing the readings of sensitive thermometers exposed in different
portions of an intense solar spectrum, he found that, beginning with the
violet end, he came to the maximum of light long before that of heat,
which lay at the other extremity, that is, near the red. By several
experiments it appeared that the maximum of illumination, _i. e._, the
yellow, had little more than half the heat of the full red rays; and
from other experiments he concluded that even the full red fell short of
the maximum of heat, which, perhaps, lay even a little beyond the limits
of the visible spectrum.

"In this case," he says, "radiant heat will at least partly, if not
chiefly, consist, if I may be permitted the expression, of invisible
light; that is to say, of rays coming from the sun, that have such a
momentum[35] as to be unfit for vision. And admitting, as is highly
probable, that the organs of sight are only adapted to receive
impressions from particles of a certain momentum, it explains why
the maximum of illumination should be in the middle of the
refrangible rays; as those which have greater or less momenta are
likely to become equally unfit for the impression of sight."

In his second paper on this subject, published in the same year,
HERSCHEL describes the experiments which led to the conc