ELKCTKON MKHOSCOPY 



17. BRorr.HTKN, J. L., el aJ., Sci. Lubric, 4, No. 



12, 23 (1952). 



18. McBrian, R., A.S.M.E., Paper 52-A40 (1952). 



19. B.\RWELL, F. T., Grunberg, L., and Scott, 



D., (Instn mech. Engrs, London) "Proc. 

 Auto Div." No. 5, p. 153 (1955). 



20. Brown, J. A., Hudson, C. N. and Loring, 



L. D., Inst. Spokesm. nat. Lubric. Gr. Inst. 

 Vol. XV, No. 11, p. 8, 1952. 



21. Milne, A. A., Scott, D. and Scott, H. M., 



(Instn. mech. Engrs) (London) "Proc. 

 Conference on Lubrication and Wear," 

 Paper 45, 450-153 (1957). 



22. Renshaw, T. a., Ind. Eng. Chem., 47, (4), 834 



(1955). 



23. Vold, M. J. and Vold, R. D., J. Inst. Petrol., 



38, No. 339, 155 (1952). 



D. Scott 



WILSKA LOW-VOLTAGE MICROSCOPE 



The first successful experiments with early 

 electron microscopes showed that almost all 

 the objects that were known from light 

 microscopy appeared more or less like sil- 

 houettes. Even bacteria were too thick to 

 be seen through. With higher accelerating 

 voltage the electrons gained more penetrat- 

 ing power. At about 200 kV the bacteria 

 became fairly translucent but because of the 

 difficulties of shielding and regulating volt- 

 ages of this range, the usual laboratory elec- 

 tron microscopes have an anode potential 

 between 50-100 kV only. 



Such a potential gives a good penetrability 

 for objects of a thickness from a few hundred 

 to a few thousand Angstrom units. The 

 resolving power of these instruments may 

 be as high as 10 A when measured using ob- 

 jects with heavy atoms. Most biological 

 objects consist of atoms of relatively light 

 weight, and of these practically no image is 

 formed even if their size is several times this 

 optimum resolution hmit. The use of heavy- 

 atom stains or relief shadowing in vacuo 

 increases the contrasts to some degree; yet 

 most of the structures in the 10-50 A region 

 have remained unseen. 



The need of more contrast became clear 



about ten years ago. Electrons of 50 kV 

 gave no hope of revealing structures that 

 one wanted to see. For this reason experi- 

 ments were begun with slower electron 

 speeds, down to 25. 10, and even 5 kV. On 

 the fluorescent screen it was easy to see how 

 the contrasts were dramatically improved. 

 Holes that were hardly observable on the 

 transparent collodion membrane at 50 kV 

 were surrounded by a pitch-black area caused 

 by the same membrane at 5 and also at 10 

 kV. The images were too unstable to be 

 photographed with sharpness, however. 

 Low-voltage electron beams are considerably 

 influenced by disturbances of magnetic stray 

 fields as well as impurities on the walls of 

 the column and at the edges of aperture 

 holes. These impurities become charged by 

 the electrons and affect the beam in an ir- 

 regular manner. Similar charges on the 

 photographic emulsion itself may affect a 

 low-voltage image much more than a high- 

 voltage one. 



For these reasons it was necessary to 

 build an entirely new instrument to meet 

 the extra demands of low-voltage work. The 

 present electron microscope is the fifth in 

 the series of trial models of "home-made" 

 low-voltage electron microscopes. All its 

 predecessors and the main parts of this 

 instrument have been built in Finland. 



The construction of the present electron 

 microscope in the United States is now 

 practically completed, too. The instrument 

 is probably the smallest of its kind, the col- 

 umn measuring only 13 inches from the 

 cathode to the fluorescent screen. It has 

 four electromagnetic lenses: condenser, ob- 

 jective, intermediate lens, and projector. 

 Withhi the objective lens circuit there is an 

 electromagnetic stigmator consisting of eight 

 minute coils. The alignment is purely electro- 

 magnetic. Both tungsten and oxide-coated 

 platinum cathodes can be used. To protect 

 against damage by positive ion bombard- 

 ment, there is an ion trap between the anode 

 and the cathode. To prevent negative 



314 



