1096 Mr. D. Coster on the Spectra of X-rays 



making use of the increased absorption in the silver of 

 the photographic plate. For this region of wave-lengths, 

 especially if the rather faint discontinuity L 3 is to be photo- 

 graphed, it is desirable to exclude totally the spectra of higher 

 order. For this reason the maximum tension on the tube 

 should be not more than twice the critical exciting tension. 

 Therefore in taking the L 8 absorption the mean tension on 

 the tube, as read with the Braun electrometer, was fixed at 

 about 5400 volts. As a fairly large current was used ( 50 m.a.) 

 a very good plate was obtained in four hours. The discon- 

 tinuities Lj and L 2 were both found on one plate after an 

 exposure of about three hours. 



§ 10. As has been stated in Part I. § 6, T have tried to 

 obtain some experimental information about the existence of 

 the line L 3 L 1? i. e., the transition Li-L 3 . For tungsten this 

 line should lie in the M-region, between the lines M/9 and 

 M7 which have been measured by Stenstrom. We may 

 calculate the wave-length of this line for W from the 

 following data. The frequency of the line L 3 Li is equal 

 to the frequency difference of the absorption discontinuities 

 In * and L 3 *. These have been measured by Duane and 

 Patterson. They found 1213'6 and 1024 X.U. respectively. 

 From this we find for the frequency difference L 3 — L x 139*07 

 in multiples of the Rydberg number, from which we may 

 calculate the required wave-length as 6553 X.U. Taking 

 into account the limits of experimental error given by 

 Duane and Patterson, we find that the error -in the wave- 

 length in question must be less than 3 per cent. We may 

 calculate the same wave-length with somewhat greater 

 accuracy in the following way. From diagram I. we 

 see that the following relation must hold between the 

 frequencies : 



L/3 2 + (L 3 - L^Lft + My. 



Putting into this formula the frequencies measured by 

 Siegbahn for L/3 2 and L/3 3 and the value for M7 measured 

 by Stenstrom, we find for the same frequency difference 

 L 3 — L x 139*05, giving the same wave-length 6553 X.U. 

 Taking into account the limits of error of the M7 line and 

 of the L-lines, which can be measured with greater accuracy 

 than the discontinuities, we find that the error in this value 

 is less than 0*3 per cent. 



We should expect that this line would only arise if an 

 electron is removed from the L-shell in such a way that the 



* By these symbols the levels as well as tlie absorption-discontinuities 

 corresponding to these levels are denoted. 



