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 same conditions.

Sec. 9. Double Refraction of Heat.

I will now abandon the Nicols, and send through the piece of Iceland spar (B, fig. 53), already employed (in Lecture III.) to illustrate the double refraction of light, our sifted beam of invisible heat.  To determine the positions of the two images, let us first operate upon the luminous beam.  Marking the places of the light-images, we introduce between N and L our ray-filter (not in the figure) and quench the light.  Causing the pile to approach one of the marked places, the needle remains unmoved until the place has been attained; here the pile at once detects the heat.  Pushing the pile across the interval separating the two marks, the needle first falls to 0 deg., and then rises again to 90 deg. in the second position.  This proves the double refraction of the heat.

[Illustration:  Fig. 53.]

I now turn the Iceland spar:  the needle remains fixed; there is no alteration of the deflection.  Passing the pile rapidly across to the other mark, the deflection is maintained.  Once more I turn the spar, but now the needle falls to 0 deg., rising, however, again to 90 deg. after a rotation of 360 deg..  We know that in the case of light the extraordinary beam rotates round the ordinary one; and we have here been operating on the extraordinary heat-beam, which, as regards double refraction, behaves exactly like a beam of light.

Sec. 10. Magnetization of Heat.