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The layer of horn used for this research was peeled in relatively large sheets off the skin after the subject had been exposed to an arc lamp. Microscopic sections of it showed several layers of hornified cells. The sample was free from holes, and the average thickness was 0.022 mm. A sample of the same origin had been used in a research by angus and Taylor on the transmission of visible and ultra-violet rays. The present paper completes the study of the transmission and is of interest in regard to the penetration of rays, from various sources of heat, through the outer horny layer of the skin. Apparatus and Method As a source of infra-red radiation shorter than 2.8 μ a 100-watt tungsten filament gas-filled lamp was used, and for radiation longer than 2.8 μ a small electric resistance made by Gerdes of New York. The pre-heater of a Nernst filament is very often used as a source, but owing to the nature of the experiments to be described the emission of this source was found to be too low for the Paschen galvanometer which could not be used at the high sensitivities of which it was capable. The dispersive system was a Hilger infra-red spectrometer (small model) with a collimator slit, collimator mirror, rock-salt prism, Wadsworth mirror, telescope mirror and telescope slit behind which the thermopile was mounted. The prism table was rotated by a wave-length drum. As apparatus of this sort does not lend itself to esceptionally refined measurements, but for the work to be described its accuracy was more than sufficient.

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