and the Theory of Atomic Structure. 1077 



smaller values of k, the removal of an electron from this sub- 

 group may therefore be expected to represent a simple and 

 well-defined reversal of a step in the process of formation of 

 the group. 



In this connexion it must be pointed out that the proposed 

 explanation of the origin of these levels requires that the 

 screening constants for a pair of levels coi responding to a 

 relativity doublet should have approximately the same value. 

 No simple explanation of this, however, is offered in the 

 present state of the theory. 



§6. Though there still remain some difficulties, we may 

 say that the X-ray spectra are built up in a simple manner 

 and that there exist many analogies between these spectra 

 and the series spectra in the visible region. There are also, 

 however, some striking differences. In the visible spectrum, 

 transitions in which the azimuthal quantum number remains 

 the same do not occur under ordinary conditions. In the 

 X-ray spectrum, however, there are several lines for which 

 k remains constant. A few of them are fairly intense lines 

 {e.g., in the case of the heavier elements L/3 4 and L/3 3 and 

 also Ly 2 and Ly 3 are of about the same intensity). Recently 

 I have found that there is another difference between the 

 X-ray spectrum and the visible spectrum. In the latter 

 there exist also transitions for which the total quantum 

 number n does not change at all. To these belong for 

 instance the first line of the principal series of the alkali 

 metals. From this we might expect that the transition 

 L x -L 3 would give rise to a line in the X-ray spectrum which 

 might easily be detected. An investigation with a tungsten 

 anticathode showed, however, that this line does not exist at 

 all, or at any rate must be very weak. Experimental 

 particulars are given in Part II. 



These various differences between X-ray spectra and 

 optical spectra need not be surprising in view of the funda- 

 mental differences which exist (in spite of analogies) between 

 the origins of the two types of spectra. This difference is 

 due to the fact that in the emission of the optical spectrum 

 we have to do with the change of the motion of an electron 

 whose orbit is characterized by higher quantum numbers 

 than the orbits of the other electrons in the atom. In the 

 emission of the X-ray spectrum, however, we meet with a 

 change in the motion of an electron which must be expected 

 to be in intimate interaction with the electrons of the same 

 shell moving in orbits with the same quantum numbers *. 



* Compare Bohr, Zeitschr.f. Physik, ix. p. 1 (1922). 



