not only where the light was at a maximum the heat
was very inconsiderable, but that where there was a maximum exhibition
of heat, there was not a trace of light.

"This consideration," he writes, "must alter the form of our
proposed inquiry; for the question being thus at least partly
decided, since it is ascertained that we have rays of heat which
give no light, it can only become a subject of inquiry whether some
of these heat-making rays may not have a power of rendering objects
visible, superadded to their now already established power of
heating bodies. This being the case, it is evident that the _onus
probandi_ ought to lie with those who are willing to establish such
an hypothesis, for it does not appear that Nature is in the habit of
using one and the same mechanism with any two of our senses. Witness
the vibration of air that makes sound, the effluvia that occasion
smells, the particles that produce taste, the resistance or
repulsive powers that affect the touch--all these are evidently
suited to their respective organs of sense."

It is difficult to see how the fallacy of this argument could have been
detected by any one not familiar with the fundamental physiological law
that the nature of a sensation is in no wise determined by the character
of the agent producing it, but only by the character of the nerves acted
upon; but, as already intimated, this law belongs to a later epoch than
the one we are considering. HERSCHEL thus finally concluded that light
and radiant heat were of essentially different natures, and upon this
supposition he explained all of the phenomena which his numerous
experiments had shown him. So complete and satisfactory did this work
appear to the scientific world, that for a long time the question was
looked upon as closed, and not until thirty-five years later was there
any dissent. Then the Italian physicist, MELLONI, with instrumental
means a thousand times more delicate than that of HERSCHEL, and with a
far larger store of cognate phenomena, collected during the generation
which had elapsed, to serve as a guide, discovered the true law. This,
as we have seen, was at first adopted by HERSCHEL on philosophical
grounds, and then rejected, since he did not at that time possess the
key which alone could have enabled him to properly interpret his
experiments.

It is well to summarize the capital discoveries in this field made by