Photoelectric Measurements of the Transmission of Fluorite in the Schumann Region

The absorption of several pieces of clear, colorless fluorite from Zell in Baden was measured in the Schumann region at ten different wave-lengths between 1600 and 1235A. A grating vacuum spectrograph was used with a photoelectric cell taking the place of the photographic plate. The relationship between the intensity of the light entering the cell and the current from the cell was found to be of the form $I=\mathrm{const}.i^z$. $Z$ was found for all the wave-lengths used. All intensity ratios were determined with an inaccuracy not exceeding one percent. It has been possible to distinguish between the light absorbed by the body of the fluorite and that removed by the surfaces. Although every precaution was taken to produce a good polish, the loss of light at the surfaces was much greater than would be expected from Fresnel's law of reflection and the index of refraction of fluorite.