Possibilities of Future Development 



109 



which could be followed by a weak lens field to bring the focal 

 length / to any prescribed value. It has been shown in the fore- 

 going section that recent developments of the uncorrected micro- 

 scope are making use of strong fields near the object in the small 

 margin which is still left for this type of instrument, but for 

 radical improvements it is of no value. We have rather to go 

 the opposite way, as illustrated in the third part of figure 39. 

 Instead of producing a very strong, short condensing lens near 

 the object, it is suggested to produce a short focus by follozving 

 up an ordinary condensing lens by a long diverging lens. 



It has been proved in chapter 11 that trajectories with an 

 inflection can never be realized with magnetic lenses, as the tra- 

 jectory always bends toward the axis. Are they also impossible 

 with electrostatic lenses? The answer fs : Not impossible, but 

 impracticable. This is illustrated in figure 40, which shows sev- 

 eral trajectories of the desired type, w^ith the corresponding lens 

 field. The lens potentials had to be drawn on a logarithmic scale 

 in order to accommodate the enormous values which are neces- 



ORDINARY MAGNSTiCLENS. 



rebsch's suggestion. 



tf p LO/s^G LENS \^flTH SHORT FOCAL LEISGTM. 

 Fig. 39. The problem of the short-focus lens 



