btained 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.

When the Nicols are crossed and the field is dark, you have seen, in
the case of light, the effect of introducing a plate of mica between
the polarizer and analyzer. In two positions the mica exerts no
sensible influence; in all others it does. A precisely analogous
deportment is observed as regards radiant heat. Introducing our
ray-filter, the thermo-pile, playing the part of an eye as regards the
invisible radiation, receives no heat when the eye receives no light;
but when the mica is so turned as to make its planes of vibration
oblique to those of the polarizer and analyzer, the heat immediately
passes through. So strong does the action become, that the momentary
plunging of the film of mica into the dark space between the Nicols
suffices to send the needle up to 90 deg.. This is the effect to which the
term 'depolarization' has been applied; the experiment really proving
that with both light and heat we have the same resolution by the plate
of mica, and recompounding by the analyzer, of the ethereal
vibrations.

Removing the mica and restoring the needle once more to 0 deg., I
introduce between the Nicols a plate of quartz cut perpendicular to
the axis; the immediate deflection of the needle declares the
transmission of the heat, and when the transmitted beam is properly
examined, it is found to be circularly polarized, exactly as a beam of
light is polarized under the sa