84 The Electron Microscope 



The Philips Incandescent Lamp Factories, Eindhoven, Hol- 

 land, have recently completed the prototype of a magnetic elec- 

 tron microscope with many novel and interesting features on 

 the basis of research extending over many years by Dr. le Poole 

 of the University of Delft. It is a desk model instrument with 

 less than three feet total length, but operated with voltages up 

 to 100 kv and with electronic magnifications not inferior to 

 most other, much longer instruments. The saving in length is 

 achieved by a third lens, i.e., a second projector lens. Another 

 interesting feature is the very large fluorescent screen, of 8 in. 

 diameter, at the end of the tube, as in the R.C.A. desk model. 

 This allows very accurate visual focusing. The film on which 

 the photographs are taken is arranged very much nearer to the 

 second projector lens, so that 'the photographs can be accom- 

 modated on 35 mm film. It is believed that with the fine grain 

 of modern photographic material this size is entirely sufficient. 

 Without the five times larger screen for visual focusing, only 

 a small proportion of the photographs taken would do justice 

 to the capacity of the film. 



The objective is of radically dififerent design from that of 

 all other microscopes. Here the suggestions, discussed later, 

 on p. 101, have been put into practice for the first time: A large 

 lens, up to a certain limit, allows a shorter focal length and 

 smaller spherical aberration than a smaller one. The bore of 

 the pole pieces is 1 1 mm, about four times larger than in almost 

 all other microscopes. The object is fixed in the center of the 

 magnetic field. This allows realizing a focal length of only 3 mm 

 at 100 kv, and by their large dimensions the pole pieces can be 

 manufactured much more precisely than smaller ones. They 

 allow also to accommodate an important new organ, an adjust- 

 able objective aperture which can be displaced axially and 

 radially. 



Another interesting novel feature is an auxiliary lens, be- 

 tween the objective and the first projector, which allows taking 

 electron diffraction diagrams, without moving the object from 

 the position in which photographs are taken. 



