Many minerals that are opaque to visible light are transparent to infrared light. By adapting photoelectric cells, sensitive to infrared, to different optical instruments used in mineralogy, it is possible to obtain the refractive indices, birefringence, absorption indices, etc. The variations of the intensity of the infrared light transmitted by the mineral can be converted by the cell into variations in electric current. It is a relatively simple task to interpret these values of the current which are recorded on a sensitive galvanometer.

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