owever, strongly
illuminated; and above it is a lens which, if permitted, would form an
image of the needle and dial upon the ceiling. There, however, it
could not be conveniently viewed. The beam is therefore received upon
a looking-glass, placed at the proper angle, which throws the image
upon a screen. In this way the motions of this small needle may be
made visible to you all.

The delicacy of this apparatus is such that in a room filled, as this
room now is, with an audience physically warm, it is exceedingly
difficult to work with it. My assistant stands several feet off. I
turn the pile towards him: the heat radiated from his face, even at
this distance, produces a deflection of 90 deg.. I turn the instrument
towards a distant wall, a little below the average temperature of the
room. The needle descends and passes to the other side of zero,
declaring by this negative deflection that the pile has lost its
warmth by radiation against the cold wall. Possessed of this
instrument, of our ray-filter, and of our large Nicol prisms, we are
in a condition to investigate a subject of great philosophical
interest; one which long engaged the attention of some of our foremost
scientific workers--the substantial _identity of light and radiant
heat_.

That they are identical in _all_ respects cannot of course be the
case, for if they were they would act in the same manner upon all
instruments, the _eye_ included. The identity meant is such as
subsists between one colour and another, causing them to behave alike
as regards reflection, refraction, double refraction, and
polarization. Let us here run rapidly over the resemblances of light
and heat. As regards reflection from plane surfaces, we may employ a
looking-glass to reflect the light. Marking any point in the track of
the reflected beam, cutting off the light by the dissolved iodine,
and placing the pile at the marked point, the needle immediately
starts aside, showing that the heat is reflected in the same direction
as the light. This is true for every position of the mirror.
Recurring, for example, to the simple apparatus employed in our first
lecture (fig. 3, p. 11); moving the index attached to the mirror along
the divisions of our graduated arc (_m_ _n_), and determining by the
pile the positions of the invisible reflected beam, we prove that the
angular velocity of the heat-beam, like that of the light-beam, is
twice that of the mirror.

[Illustration: Fig. 49.]