The action of the arrangement is this:  the heat, falling on the pile, produces an electric current; the current, passing through the coil, deflects the needles, and the magnitude of the deflection may be made a measure of the heat.  The upper needle moves over a graduated dial far too small to be directly seen.  It is now, however, 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.]

As regards reflection from curved surfaces, the identity also holds good.  Receiving the beam from our electric lamp on a concave mirror (m m, fig. 49), it is gathered up into a cone of reflected light rendered visible by the floating dust of the air; marking the apex of the cone by a pointer, and cutting off the light by the iodine solution (T), a moment’s exposure of the pile (P) at the marked point produces a violent deflection of the needle.