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.

The common reflection and the total reflection of a beam of radiant
heat may be simultaneously demonstrated. From the nozzle of the lamp
(L, fig. 50) a beam impinges upon a plane mirror (M N), is reflected
upwards, and enters a right-angled prism, of which _a_ _b_ _c_ is the
section. It meets the hypothenuse at an obliquity greater than the
limiting angle,[23] and is therefore totally reflected. Quenching the
light by the ray-filter at F, and placing the pile at P, the totally
reflected heat-beam is immediately felt by the pile, and declared by
the galvanometric deflection.

[Illustration: Fig. 50.]

Sec. 7. _Invisible Images formed by Radiant Heat._

Perhaps no experiment proves more conclusively the substantial
identity of light and radiant heat, than the formation of invisible
heat-images. Employing the mirror already used to raise the beam to
its highest state of concentration, we obtain, as is well known, an
inverted image of the carbon points, formed by the light rays at the
focus. Cutting off the light by the ray-filter, and placing at the
focus a thin sheet of platinized platinum, the invisible rays declare
their presence and distribution, by stamping upon the platinum a
white-hot image of the carbons. (See fig. 51.)

[Illustration: Fig. 51.]

Sec. 8. _Polarization of Heat_.

Whether radiant heat be capable of polarization or not was for a long
time a subject of discussion. Berard had announced affirmative
results, but Powell and Lloyd failed to verify them. The doubts thus
thrown upon the question were removed by the experiments of Forbes,
who first established the polarization and 'depolarization' of heat.
The subject was subsequently followed up by Melloni, an investigator
of consummate ability, who sagaciously turned to account his own
discovery, that the obscure rays of luminous sources are in part
transmitted by black glass. Intercepting by a plate of this glass the
light from an oil flame, and operating upon the transmitted invisible
heat, he o