Radio age research, manufacturing, communications, broadcasting, television (1941)

particle from another—is from 50 to 100 times greater than that of the ordinary light microscope. Re- solving power increases in propor- tion to the frequency of the source of illumination. Thus the resolving power of a light microscope is lim- ited by the available frequencies of visil)le illumination. Electrons, as used in electron microscopy, have wavelengths which may be 100,000 times shorter than the wavelengths commonly employed in light micros- copy. The theoretical resolution obtainable with electrons has not by any means been realized. How- ever, as in all science, perfection is only a goal, not a realization, and tomorrow's electron microscope will seek and find more than yester- day's. Magnifications Compared The very high resolution of the electron microscope enables the in- strument to work at useful mag- nifications that would have seemed incredible a few years ago. Useful magnification is the largest product of instrumental and photographic magnifications beyond which no further information is i-evealed to the eye. For the sake of compari- son, the useful magnification of a light microscope does not exceed 2000 times, while the useful mag- nification of an electron microscope in most cases exceeds 50,000 times and may reach better than 100,000 times, depending on the type of material under e.xamination. The new Universal Electron Mi- croscope provides three distinct services: straight two-dimensional microgi'aiihs: three-dimensional or stereo-micrograjihs: and diffrac- tion patterns of crystalline sub- stances. Two-dimensional pictures may be taken of particles, suspen- sions, films, crystals, etc., within the limits of the penetrating power of the electron beam, which is about 1 micron (1 25.000 of an inch). For those materials which are opaque to the electron beam—mate- rials thicker than about 2 microns —valuable information may be ob- tained through the use of the "replica technique" ichich was de- veloped for this purpose. This tech- nique consists in casting a very thin film on the surface or area to be studied, stripping the film from the specimen, and photographing the film itself. The difference in thickness of the stripped film, caused by the irregularities of the surface of the specimen, cause the electron beam to register them and produce a facsimile of the original. Refinements of this technique have led to replica micrographs possess- ing resolution almost equal to standard micrographs. Great Depth of Focus Stei-eo-micrographs are produced by taking two exi)osures of a given field. One exposure is made with the specimen tilted at a slight angle with respect to the electron beam while the second exposure is taken at an equal but directly opposite angle. When the two exposures are aligned properly and examined with suitable stereo viewers, third di- mensions are readily observed and measured. Stereo-micrographs can be made of either standard micro- graphs or replicas. In appraising the extreme usefulness of electron stereo-micrography, it should be pointed out that electron micro- scope images possess a tremendous A.\ ELECTRON MICROGRAPH OF THE SCALES OF A MOSQUrTO WING MAGNI- FIED 16.000 TIMES. depth of focus—some 15 to 25 mi- crons - - whereas light microscope images have a depth of focus of something less than 1 micron for low magnifications and are pi'wpor- tiouately shallower as magnifica- tion is increased. From the above it is apparent that the entire fields of electron stereo images are prac- tically always in complete focus and the most intimate knowledge of bulk dimensions, arrangements and linkages are easily obtainable. Used in Surface Studies F^lectron diffraction i)rovides a means for studying the atomic geometry of crystalline materials. Electron and X-i-ay diffraction studies supplement each other in that electron diffraction is used for the study of surfaces and thin films and X-Ray for the thicker mate- rials. This is brought about be- cause of the difference in penetrat- ing power of the two types of radiation employed. There are two techniques used to obtain patterns in electron diffraction studies; transmission method and refiection. Crystals in powder form are stud- ied by the transmission method and, in these cases, powder smears oi- groupings are not opacpie to the electron beam. Electrons simply are allowed to pass through the devious lattice structures of the random mounted crystals to pro- duce a pattern. For those sub- stances which are opaque or in bulk form, the reflection technique is used. In these cases the electrons are allowed to graze the specimen surface at a slight angle. Mechan- ical means are provided to properly orient the opaque specimens so that the crystal lattices are in proper relation to the electi'on beam. Kk'cti'on diffraction patterns are sometimes referred to as the "thumbprints" of the crystal uni- \erse. No two substances produce the .same pattern. In practice, dif- fraction patterns are obtained to permit the study of the atomic geometry of a new substance or to identify an unknown substance by means of its i)attern, checked against its thumbprint in file. The relative degree of crystallinity of a given substance is a further valu- able contribution portrayed by dif- [10 RADIO AGE]