possibilities of Future Development 123 



2 I ^^- ^^A 2 (51) 



■i (eK^ 

 b^U=^ . \8mc2 

 The second factor depends on the magnetic field and the space 

 charge which in turn depends on H and on the potential of the 

 sheath electrode *'A." As there is no current flowing to ''A," 

 and as the time constant of the second coil can be made very 

 long, it appears possible to keep the second factor constant within 

 much finer limits than the first. <I»o is the driving voltage V, only 

 expressed in electrostatic units instead of in volts. We obtain, 

 therefore, for the relative fluctuations of b which are produced 

 by inconstancy of the driving voltage 



^ = ^ (52) 



The meaning of this is explained in figure 44. If the driving 

 voltage increases by AV, the point "O" will still remain in focus, 

 provided that A\^ produces such a change in the first lens that 

 it forms now an image of "O" shifted to the right by Ab, as given 

 by equation (52). If the first lens were all electrostatic, the shift 

 would be zero, if it were all magnetic, equation (52) would lead 

 to inconvenient dimensions. This is the reason why it is sug- 

 gested to make the first lens a combined lens. Application of the 

 elementary lens equation, combined with the fact that the focal 

 length of a magnetic lens is proportional to V gives 



Ac _ A/,„ _ AV (53^ 



c== U- Vf 



where /m is the focal length of the magnetic part of the first lens. 

 As Ac = Ab, we obtain by combining (53) with (52) 



/,„ = ^ (54) 



By itself this would be a somewhat weak lens, it therefore ap- 

 pears preferable to add a unipotential electrostatic lens to it. 

 This complication could be avoided if it should prove possible to 



