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 obtained effects of polarization, far exceeding in magnitude those which could be obtained with non-luminous sources.  At present the possession of our more perfect ray-filter, and more powerful source of heat, enables us to pursue this identity question to its utmost practical limits.

[Illustration:  Fig. 52.]

Mounting our two Nicols (B and C, fig. 52) in front of the electric lamp, with their principal sections crossed, no light reaches the screen.  Placing our thermo-electric pile (D) behind the prisms, with its face turned towards the source, no deflection of the galvanometer is observed.  Interposing between the lamp (A) and the first prism (B) our ray-filter, the light previously transmitted through the first Nicol is quenched; and now the slightest turning of either Nicol opens a way for the transmission of the heat, a very small rotation sufficing to send the needle up to 90 deg..  When the Nicol is turned back to its first position, the needle again sinks to zero, thus demonstrating, in the plainest manner, the polarization of the heat.