1102 Mr. D. Coster on the Spectra of X-rays 



We should expect the line Me to disappear at the same time 

 as the lines L/3 5 and Ly 6 , as the existence o£ this line is also 

 bound to a 5 3 subgroup of electrons. This line has as yet 

 not been studied systematically. It has been measured by 

 Stenstrom * for U and Th and by K archer | for the elements 

 Bi, Au,and Pt. 



§ 4. The appearance of the lines Ma 1? M« 2 , and M/3 is 

 connected with the existence of 4 4 electrons in the N-shell. 

 According to Bohr, these electrons occur for the first time 

 in the rare earths. For the elements Dy (66) there must 

 already be electrons present in 4 4 orbits, as the appearance 

 of the lines Ma (a x and a 2 could not be separated for the 

 elemmts with lower atomic number than Tl) and M/3 has 

 been established by Stenstrom for this element. The wave- 

 lengths of these lines are respectively 9509 and 9313 X.U. 

 It would be of interest to try to find these lines also for 

 elements with lower atomic number than Dy. 



§ 5 The existence of the line Iyy 4 depends on the presence 

 of 5 2 electrons in the O-shell. As pointed out to me by 

 Professor Bohr, it appears from comparison with the optical 

 spectra of Cd and In, that 5 2 orbits appear for the first time 

 in the neutral atom of the latter element. The experimental 

 results seem to indicate that for the line y 4 there are certain 

 complications. The very striking changes in the intensities 

 of the pairs of lines j3 5 -y 6 (see Part III. § 3) and /3 2 _ 7i ( see 

 Part III. § 6) cannot be questioned. But usually it is very 

 difficult to make any definite assertions concerning changes 

 of the relative intensity of the lines with different elements, 

 as we are not certain that they have been exposed under the 

 same conditions. However, there is evidently a considerable 

 change in the intensity of the line hji between the elements 

 Sb and Sn. Down to In (49j, this line could be measured 

 but here it had become very faint. For Cd, where we 

 according to Bohr should expect y 4 to disappear, no certain 

 information about this line could be obtained from the experi- 

 ments. Extrapolating according to Moseley's law we find 

 for the wave-length of y 4 for Cd about 3081 X.U.. which is 

 at the same time about twice the wave-length of the copper 

 K« 2 line. Using an X-ray tube of brass and working with 

 a fairly high tension which appeared to be necessary in 

 photographing the very weak lines, it is impossible to avoid 

 wholly the characteristic Cu-radiation. Actually the copper 



* Stenstrom, Dissertation, Lund, 1919. 



t Karcher, Phys. Rev. II. xv. p. 285 (1920). The line Me has been 

 called M73 by Karcher. 



