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Updated: May 21, 2025
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.
Now, the iodine, which proves itself thus hostile to the light, is perfectly transparent to the ultra-red emission with which we have now to deal. It, therefore, is to be our ray-filter.
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. Polarization of Heat. Whether radiant heat be capable of polarization or not was for a long time a subject of discussion.
Cutting off the light by our ray-filter, and exciting the magnet, the needle is instantly deflected, proving that the magnet has opened a door for the heat, exactly as in Faraday's experiment it opened a door for the light. Thus, in every case brought under our notice, the substantial identity of light and radiant heat has been demonstrated.
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. 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.
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.
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. 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.
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