Direct Recording of Spectra in the Region 12μ to 3μ Using the Lead Sulfide Photo-Conductive Cell*

The problems arising in the use of the lead sulfide cell for direct recording of spectra have been investigated using a spectrograph designed especially for this work. The inhomogeneity of the surface of these cells, as compared with photo-multipliers, makes for important differences in their use. Using a sharply tuned amplifier, the sensitivity of the cells at room temperature is adequate to permit a resolving power of the order of 40,000 at 1.4μ. Wave-lengths may be determined to about 0.1A, using overlapping orders.