Precision-measurements in the X-Ray Spectra, 605 



where the lattice-constant of rock-salt has been taken at 

 2-81400 . lO" 8 , log 2^ = 0-7503541, and of calcite at \og2d 

 = 0-7823347 (see below). 



B. The lattice-constant of calcite. 



Bragg in his eminent work ' X-Rays and Crystal Structure * 

 recommends the use of calcite instead of rock-salt for X-ray 

 investigations on account of its more regular reflexions. 

 A curve of the intensity of reflexion produced by rock-salt 

 given in the book mentioned clearly shows what bad images 

 are obtained in some cases by these crystals. In spite of 

 this fact, experience from many hundred X-ray spectrograms 

 has taught me that with selected specimens (I have used 

 rock-salt from Stassf urt) perfectly good images with spectro- 

 graphs of this kind may be obtained. Only at very large 

 angles do somewhat better results with calcite seem to come 

 out, probably owing to the greater absorption in calcite which 

 reduces the thickness of the reflecting layer. 



The extending use of the calcite for X-ray spectrography 

 makes it desirable for one to know the relation between the 

 lattice-constant of calcite and that of rock-salt with sufficient 

 accuracy. By means of the above method for determining 

 the reflexion-angles for monochromatic X-rays this is easily 

 effected. The determination has been carried out with the 

 following three characteristic radiations: — 



Cu K«, 



Fe Ku, 



Sn L a. 



With each of them a number of spectrograms with re- 

 peated adjustments have been taken. The results are given 

 in the following table: — 



\. log 2d d.lO- 11 . 



Cu 1537-36 0-7823339 3029-03 



Fe 1932-39 0-7823386 302907 



Sn 359294 0-7823327 302902 



which gives us the most probable value for the lattice- 

 constant of calcite (cleavage-face) 



log 2^=0-7823347, 



where the value for rock-salt is taken as 

 log 2^=0-7503541, 



