490 Dr. H. S. Allen on Electronic 



same constancy as those in Table L, and in themselves could 

 not be regarded as sufficient to justify the proposed relation. 

 The mean value of v B obtained from these results is 

 -3*31 x 10 15 sec. -1 , which is not far from the Rydberg value. 



In the case of sodium Compton and Richardson found two 

 maxima in the sensitiveness frequency curve. The first 

 maximum at 3()0/a//, presumably corresponds to the maximum 

 of the iC selective " effect observed by Pohl and Pringsheim 

 at 340/^yLt. The second maximum at 227/x//, is supposed to 

 correspond to the " normal " effect. The value of l$v deduced 

 from the second maximum is 4Jx 3"26 x 10 15 , so that the 

 frequency number (4 J) is half as large again as the frequency 

 number (3) for the " selective " effect. 



Mention must also be made of a- paper by Souder *, who 

 worked in Prof. Millikan's laboratory. He states that the 

 maximum suggesting the " selective " effect is found under 

 conditions which are supposed to yield only the " normal " 

 effect. No figures are given for the wave-length corre- 

 sponding to the maximum, but from the curves published in 

 the paper the wave-length in the case of sodium appears to be 

 about 350 /a//,, which agrees with the results already recorded. 

 For lithium, the maximum is somewhere in the neighbourhood 

 of the wave-length, \=280 /-tju,, given by Pohl and Pringsheim. 

 With a freshly-cut surface of potassium, however, the maxi- 

 mum appears at about 380/a/a, which differs considerably 

 from Pohl and Pringsheim'' s value, 435 fju/j,. From Souder's 

 maximum the value of Ni> is found to be 4^x 3*33 x 10 15 , 

 indicating a change in the , frequency number, n, from 

 4 to 4± 



§ 3. The Limiting Frequency of the Photoelectric Effect. 



It has been established by the results of a number of in- 

 vestigators that the photoelectric effect ean be observed only 

 when the wave-length of the exciting light is less than a 

 certain critical value — the long- wave-length limit. Thus for 

 the emission of electrons to take place the frequency must 

 exceed the limiting frequency, v . Einstein f suggested that 

 the energy of the electron liberated by light of frequency v 

 could be expressed in the form 



± mv 2 = Ye = hv — hv . 



Here V denotes the potential necessary to prevent the 

 emission of an electron (charge e, mass m), and A is Planck's 



* Souder, Phys. Rev. vol. viii. p. 310 (1916). 



t Einstein, Ann. d. Physik, vol. xvii. p. 132 (1905). 



