and the Theory of Atomic Structure. 1097 



remaining electrons form a configuration corresponding to 

 a L 3 -level. Using a tungsten anticathode, the tension on the 

 tube must according to Einstein's relation be more than 

 12000 volt. As the expected line might not be very strong, 

 it is desirable to work with a tension which is at least twice 

 as great. The tube was therefore driven wdth the maximum 

 tension which could be obtained. With this tension 51 very 

 strong L-spectrum in the first order was obtained in ten 

 minutes. As analysing crystal a gypsum crystal was used ; 

 the time of exposure was one hour. Besides the lines M/3 and 

 M7 several other lines which could not be identified at first 

 sight were found on the plate. Now the lines in the M-series 

 have a very typical structure, being rather diffuse and 

 broadened on the short wave-length side. Therefore no 

 doubt could arise as to the identification of these lines. As 

 to the new line LgLx, however, we should expect that it 

 would be distinctly different, and especially that it would be 

 fairly sharp for W. For a right interpretation of the plate 

 it was therefore desirable to exclude all the lines which 

 appeared on the plate in higher order. This could be done in 

 the following simple way. A new plate was exposed under 

 the same conditions. Half of this plate was covered with 

 an aluminium sheet ol 7 fi thickness. By this sheet wave- 

 lengths of more than 6000 X.U. are totally absorbed. Fig. 4 

 (PL XXIII.) gives a reproduction of the spectrum obtained 

 in this way. Most of the lines are not appreciably absorbed 

 by the aluminium, but the lines M/3 and My have been totally 

 absorbed. Copper and tungsten lines w r ere found in 4th and 

 5th order. Between the L/9 2 and L/^ line of W the line 

 L/3 3 w r as seen on the plate also in 5th order. This line comes 

 from the L 3 level. As L/3 3 is not a very strong line for W, 

 it was very faint on the plate, and it cannot be seen on the 

 reproduction. Further the Ca K absorption line in the 

 second order may be seen as a dark line quite near the M7 

 line of W. This line is due to the selective absorption in tha 

 Ca of the crystal*. From the appearance of the tungsten 

 lines in such high order we may conclude that the experi- 

 mental conditions were such as to excite the L 3 level quite 

 sufficiently to give rise to the line L 3 L l5 unless this transition 

 is very improbable. The place where we should expect the 

 line is marked with a j on the reproduction. In the neigh- 

 bourhood of this place only one very faint line was seen on 

 the plate. This line, however, was not appreciably absorbed 



* In general, absorption discontinuities are obtained in a very easy 

 manner if the absorbing element forms part of the analysing crystal. 



