High-Frequency Spectra of the Elements. 707 



cannot be avoided. Unfortunately, when photographed it 

 takes the form of irregular fringes, which effectually hide 

 faint spectrum-lines. A change of target, with the position 

 of slit and crystal unaltered, does not affect the appearance 

 of the fringes, a fact which proves that they are due to the 

 general heterogeneous radiation. It is easy to show that the 

 fringes are merely a very foreshortened pattern of patches 

 on the crystal surface which reflect exceptionally well. The 

 way in which they move and spread out laterally as the 

 crystal is turned provides a proof of this, and so does 

 Barkla's * observation that when the crystal is moved side- 

 ways the fringes move with it. It is easy to devise methods 

 for getting rid of the fringes. In the first place, narrowing 

 the slit or increasing the distance from the crystal will diminish 

 their intensity compared with that of the line-spectrum. 

 In the second place, turning the crystal will move and blur 

 the fringes, but leave the sharpness of the lines unaffected 

 provided the slit and photograph are equidistant from the 

 reflecting surface |- The quantitative measurements of 

 Moseley and Darwin ± on the reflexion of the general 

 radiation must have been little affected by these fringes, as 

 the incident beam was restricted to a very narrow pencil 

 which always impinged on the same part of the crystal. 



The results obtained for radiations belonging to Barkla's 

 K series are given in Table I., and for convenience the 

 figures already given in Part I. are included. The wave- 

 length X has been calculated from the glancing angle of 

 reflexion by means of the relation n\ = 2dsm6, where 

 d has been taken to be 8*454 x 10~ 8 cm. As before, the 

 strongest line is called a and the next line /3. The square 

 root of the frequency of each line is plotted in fig. 3, and 

 the wave-lengths can be read off with the help of the scale 

 at the top of the diagram. 



The spectrum of Al was photographed in the first order 

 only. The very light elements give several other fainter 

 lines, which have not yet been fully investigated, while the 

 results for Mg and Na are quite complicated, and apparently 

 depart from the simple relations which connect the spectra 

 of the other elements. In the spectra from yttrium onwards 

 only the a line has so far been measured, and further results 

 in these directions will be given in a later paper. The 



* Barlda and Martyn, Proc. Phys. Soc. London (1913). 

 f Moseley, loc. cit. p. 1025. See also W. H. and W. L. Braggj Proc. 

 Roy. Soc. A, lxxxviii. p. 428 (1913). 

 X Moseley and Darwin, loc. cit. 



