lusion given
above. This paper contains a remarkably interesting passage which
admirably illustrates HERSCHEL'S philosophic method.

"To conclude, if we call light, those rays which illuminate objects,
and radiant heat, those which heat bodies, it may be inquired
whether light be essentially different from radiant heat? In answer
to which I would suggest that we are not allowed, by the rules of
philosophizing, to admit two different causes to explain certain
effects, if they may be accounted for by one. . . . If this be a true
account of the solar heat, for the support of which I appeal to my
experiments, it remains only for us to admit that such of the rays
of the sun as have the refrangibility of those which are contained
in the prismatic spectrum, by the construction of the organs of
sight, are admitted under the appearance of light and colors, and
that the rest, being stopped in the coats and humors of the eye, act
on them, as they are known to do on all the other parts of our body,
by occasioning a sensation of heat."

We now know that the reasoning and conclusion here given are entirely
correct, but they have for their basis only a philosophical conception,
and not a series of experiments designed especially to test their
correctness. Such an experimental test of this important question was
the motive for a third and last paper in this department of physics.
This paper was published in volume ninety of the _Philosophical
Transactions_, and gave the results of two hundred and nineteen
quantitative experiments.

Here we are at a loss to know which to admire most--the marvellous skill
evinced in acquiring such accurate data with such inadequate means, and
in varying and testing such a number of questions as were suggested in
the course of the investigation--or the intellectual power shown in
marshalling and reducing to a system such intricate and apparently
self-contradictory phenomena. It is true that this discussion led him to
a different conclusion from that announced in the previous paper, and,
consequently, to a false conclusion; but almost the only escape from his
course of reasoning lay in a principle which belongs to a later period
of intellectual development than that of HERSCHEL'S own time.

HERSCHEL made a careful determination of the quantitative distribution
of light and of heat in the prismatic spectrum, and discovered the
surprising fact that