Spectra of Helium , 8,-c. in the bltra- Violet. 293 



effect observed is due to radiation corresponding to radii 

 near the centre o£ the target, provided that the radiation is 

 completely heterogeneous between the limits corresponding 

 to the edges of the slits. It is to be observed that no 



Fi* 3. 



4 5 6 



electrons can enter the cup on radii shorter than the inside 

 radius of the slits, so that when the field is raised to the 

 elimination of any particular spectrum, we are concerned 

 only with this inner radius in calculating the limiting fre- 

 quency. Since we are chiefly considering in this paper the 

 determination of maximum frequencies, the energy distribu- 

 tion across the slit for any one setting is not of immediate 

 interest for our present purposes. 



The limiting frequencies received in the cup at successive 

 values of the field for the slit width (*16 cm.) used in the 

 apparatus are shown in fig. 4. The solid line given for each 

 field value shows the total range of frequencies received. 

 The dotted lines represent the same limits expressed in 

 wave-lengths. The slit was purposely made very wide in 

 this apparatus. The limits can consequently be very con- 

 siderably narrowed by using a narrower slit. 



The calculations are made in the following way. At any 

 given setting of the magnetic field the velocity v of the 

 photoelectrons travelling on any given radius r can be deter- 

 mined from the relation 



v — — rH. 

 m 



