Precision-measurements in the X-Ray Spectra. 641 



the plane through the two slits. This adjustment can he 

 controlled by photographing a monochromatic ray on both 

 sides of the direct ray. For this purpose, firstly, the plate 

 is exposed for the direct pencil through the two slits for some 



JKff. 3. 



seconds; then the box is turned through an angle approxi- 

 mately equal to the reflexion-angle of the monochromatic 

 ray, first on one side, then on the other. Two screens of 

 lead immediately before the photographic plate serve to 

 prevent blackening of the plate through the direct pencil. 

 When the adjustment is good the spectral lines on the 

 two sides are at the same distance from the slit-image. 

 As to the dimensions of the spectrograph, it may be 

 mentioned that the distance from the slit to the plate was 

 about 500 mm. This distance, which was measured with 

 great accuracy, and the distance between the lines on the 

 plate, are sufficient for the evaluation of the wave-lengths. 

 The lattice-constant of calcite, which was only used for the 

 K-series, was taken to be log 2d = '7823347, as determined 

 in Fart I. of this paper. 



The strongest line, Ka b of the K-series was determined 

 with great care in the way just mentioned and with two 

 different specimens of calcite crystals. The two groups 

 of determinations show a little difference in the mean, 

 but this can hardly be ascribed to a difference in the 

 lattice-constant but may be caused by accidental Eaults in 



