Exception to the Principle of Selection. 661 



ordinary arc lines (principle, 1st subordinate, 2nd sub- 

 ordinate, Bergmann series) are excited, as shown by the 

 upper spectrogram. On increasing the current to 1000 

 milliamperes, all other conditions remaining identical, ihe 

 pair A, 4641 and X 4:64:2, entirely absent at 70 milliamperes, 

 now become one oil the strongest pairs in the spectrum, as 

 shown in the lower half of fig. 2 (PL XIII.). These lines have 

 the spectral notation ls-3d and are accordingly, if Sommer- 

 feld's interpretation be accepted, the result of an interorbital 

 transition in which the azimuthal quantum number changes 

 by two units. Similar results were obtained with sodium. 



The fact that the lines arise when the exciting field is only 7 

 volts, and in a location well shielded electrostatically from even 

 this small field, makes it extremely doubtful if this and the 

 other above mentioned exceptions to the principle of selection 

 may be attributed to the incipient Stark effect of the applied 

 electrostatic field. The effect in general appears closely 

 related to the magnitude of the current, and might therefore 

 be of a magnetic nature. However, the magnetic effect of 

 a projected beam of electrons should not be large within the 

 beam itself, from which the radiation is observed. A 

 possible explanation may lie in the influence of the electric 

 field of neighbouring ions and electrons, the concentration of 

 which may increase with the current. On the other hand, 

 the explanation may involve a reconsideration of the method 

 whereby single azimuthal quantum numbers have been 

 assigned to each of the s,p, d, and b terms. 



Summary. 

 A new form of discharge tube is described in which the 

 applied electrostatic field can exert no influence upon the 

 radiation. Using this tube, the lines ls-?>d w T ere observed to 

 be one of the most intense pairs in the potassium spectrum, with 

 a current of 1000 milliamperes, while at 70 milliamperes their 

 presence was not detected. The excitation of these lines, 

 for which the change in azimuthal quantum number according 

 to Sommerfeld is two units, is contrary to Rubinowicz's 

 principle of selection as extended by Sommerfeld. It cannot 

 be attributed to the Stark effect of the applied exciting 

 potential. This fact suggests that the extra components in 

 the fine structure of the helium lines likewise may not be 

 called out by the applied field. 



In conclusion, the writers desire to express their apprecia- 

 tion for the suggestions made by Dr. L Silberstein, who very 

 kindly read the manuscript. 



Bureau of Standards, Nov. 8, 1921. 



