ELECTRICAL RESISTANCE 



85 



ELECTRONi'MICROSCOPY 



esting, as Danielli remarks (Bourne. 



p. 42), that a definite pattern slioula 



emerge of a cell plasma membrane only 



10~^-10~'' cm. in thickness corresponding 



to a specific resistance of lO'^-lOi^ ohms. 



Electron Microscopy. Details provided bj' 



Dr. W. L. Simpson of The Barnard Free 



Skin and Cancer Hospital. 



L Transmitted electron beam type. 



The relationship of resolving power 



(R.P.) to the wave length (A) of light 



employed and to the numerical aperture 



(N.A.) of a lens system as expressed in 



5\ 

 the relation R.P. equals t^ i- proved for 



many years an apparently insurmount- 

 able limitation to the biologist's desire 

 to investigate directly minute structure 

 of cells and tissues. Even when ultra- 

 violet light of 2250A was employed the 

 limit of resolution was O.OS/u in a system 

 of N.A. L40. With visible light the 

 limit was approximately 0.2/i. On the 

 assumption that the angle of visual 

 acuity is 1 minute of an arc, the greatest 

 magnification that was practical with 

 visible light ranged from 1750 to 2100 

 times. There is, of course, no limitation 

 to the actual magnification tliat may be 

 achieved. Increases beyond the limit 

 mentioned, however, do not reveal new 

 structures. As long as this was true 

 there seemed no hope of direct confirma- 

 tion of the amazing findings made 

 possible by such new methods as x-ray 

 diffraction, ultracentrifugation studies, 

 chemical studies of virus structure, and 

 polarized light methods. 



Small wonder is it then that the 

 biologist has grasped with enthusiasm 

 at the possibilities of visualizing ultra- 

 microscopic structure by means of 

 devices that have grown from the fertile 

 field of electron optics. Of most general 

 interest is the electron microscope. 

 With this instrument, using the same 

 equation for resolving power, it should 

 be possible to reach a resolution of at 

 least 0.00 l/i. Thus, an improvement of 

 at least 200 times over the limit with 

 visible light might be achieved. The 

 practical limit on magnification has been 

 placed at from 70,000 to 100,000 times. 



Historically the electron microscope is 

 now fifteen years old. Busch described 

 the first such instrument using magnetic 

 lenses (Busch, H., Archiv. f. Elektro- 

 teknik, 1927, 18, 583-594). Though 

 many improvements were m^ade in de- 

 sign it was not until 10 years later that 

 the instrument reached the point of 

 being of practical use. Chiefly through 

 the work of Ruska and Borries (numer- 

 ous papers, 1934-1940) the instrument 

 was developed to the state that it is in 

 at present. In this country an excellent 

 instrument, capable of giving high 



resolution has been developed and com- 

 mercially marketed by the Radio Cor- 

 poration of America. The apparatus is 

 described by Zworykin (V. K., Science, 

 1940, 92, 51-53). In this instrument 

 electrons emitted from a hot wire fila- 

 ment are accelerated by a potential of 

 30,000 or more volts. This beam is 

 condensed and passed through the 

 object which is carried on a wire screen 

 supported nitro-cellulose film. The 

 transmitted electron beam is focussed 

 in a greatly enlarged image by means of 

 two magnetic lenses. The image can be 

 seen on a fluorescent screen or photo- 

 graphed on a sensitive plate. 



Much work has already been reported 

 on the use of this instrument. Though 

 it is too early to evaluate these new 

 findirigs, it appears that some of them 

 contribute greatly to our knowledge of 

 the finer structure of viruses (Green, 

 R. H., Anderson, T. F. and Smadel, 

 J. E., J. Med. Research, 1942, 75, 651- 

 656) and biological fibers, e.g., studies 

 on collagen fibers by Scott and Anderson 

 (G. H. and T. F., Anat. Rec, 1942, 82, 

 445) and Schmitt, Hall and Jakus (F. 

 O., C. E., and M. A., J. Cell, and Comp. 

 Physiol., 1942, 20, 11-33). On the other 

 hand, much of the work appears to be 

 devoted to a simple attempt to see 

 things magnified more than has hitherto 

 been possible. Considerable experience 

 will undoubtedly be required before the 

 full range of usefulness of such an appa- 

 ratus can be realized. By modifications 

 in standard microtomes and special 

 treatment of tissues sections can be cut 

 0.1m thick (Richards, A. G., Jr., Ander- 

 son, T. F., and Hance, R. T., J. Exp. 

 Biol. & Med., 194-L 51, 148-152). For 

 study of sperm, see Baylor, M. R. B., 

 Nalbandov, A. and Clark, G. L., Pro. 

 Soc. Exp. Biol. & Med., 1944, 54, 229- 

 232. Recent developments, see Wyc- 

 koff, R. W. G., Science, 1946, 104, 21-26. 

 The R. C. A. and General Electric Ma- 

 chines are compared bv Click, D., Ann. 

 Rev. Biochem. 1944, 13, 705-734. 



2. Emission electron type. The earli- 

 est description of an electron micro- 

 scope in this country was of an entirely 

 different type from the new R.C.A. mi- 

 croscopes that give such prodigious 

 magnifications. McMillan and Scott 

 (J. II. and G. H., R.S.I., 1937, 8, 288- 

 290) published an account of an electron 

 microscope of simple design that used 

 as a source the electron emission of 

 heated sections of tissues. These were 

 accelerated by a potential of 1000 to 2000 

 volts, focussed by a magnetic lens onto 

 a fluorescent screen. An improved 

 design (Scott, G. H. and Packer, D. M., 

 Anat. Rec, 1939, 74, 17-29) makes pos- 

 sible magnifications of at least 150 



