Studies in the infra-red region of the spectrum. Part II.—Calibration of prism spectrometer; General procedure; and preparation of pure ammonia, phosphine and arsine

Abstract

The instrument used was a No. 2 Hilger infra-red spectrometer fitted with a "wave-length drum." It soon became evident that the wave-length drum required calibration on account of the chance of periodic errors in the screw thread. If the screw remains constant, it would be better to have the drum marked in angles which could be referred to the refractive indices of the material of the respective prisms used. As any one prism varies in the refractive index with temperature, reading of wave-length on a wave-length drum can be accurate only for one temperature. A plane mirror was mounted vertically at the centre of rotation of the prism table and an illuminated scale set up parallel to it exactly 5 metres away. The image of the scale in the mirror was observed by means of a fixed telescope with cross wires in the eyepiece. Zero was taken as 0∙5893 on the drum, which was set successively on about 80 of its graduation marks and the scale image coinciding with the cross wires noted for each setting. The displacement of the image in millimetres in each case was thus observed, and as the distance from the scale of rotation and thence the angle itself was obtained within 2 seconds of arc. Three observers agreed closely in their observations, from which a curve was drawn connecting drum calibration marks with angle of rotation (first graph).