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

instrument has a depth of focus of more than ten microns (.0004 inches) as compared to an oil imer- sion lens on a lijrht microscope which has a depth of focus of some- where in the order of .08 microns (.0000032 inches). A practical ex- ample of the e.xtreme depth of focus is illustrated in a micrograph of aluminum oxide monohydrate where two of the crystals are standing on end and one on its side, the total field being in focus. This naturally lends itself to third dimensional studies by stereoscopic pictures and so a specimen holder was designed and applied to the instrument. In the beginning, there was one standard specimen preparation wherein a 200 mesh etched screen was used to support .01 micron (.0000004 inches) collodion film. This film was prepared by floating a small drop of 2 per cent collodion dissolved in amyl-acetate on water, placing the screen on this film and picking the screen with its film uji by means of a specially designed hook. Upon this film was placed the specimen solution. This method was suitable for ordinary work covering particle study and bacteria. How- ever, this placed great limitations on the fields of application of the instrument and studies were made into the matter of various specimen preparations. Since that time, many different plastics have been applied as specimen supports. Other meth- ods have utilized mixing the speci- men directly in the film material so as to obtain better dispersion. Con- CRYST.ALS OF ALUMINUM OXIDE .MONO- HYDRATE, AS SEEN BY THE ELECTRON MICROSCOPE. rT^ 4 « « « 4 « * THE "STREAMLINED" DESIGN RECENTLY CONCEIVED FOR THE STANDARD MODEL RCA ELECTRON MICROSCOPE. siderable and detailed work has cen- tered about obtaining better dis- persion, since the finer the particles the more difficult becomes this prob- lem. Many new and interesting techni(|ues have been developed. In the study of bacteriology, there has been considerable discus- sion as to the effects of the electron beam and the vacuum upon the bac- teria themselves. Obviousl.v the bacteria cannot be studied in mo- tion since the vacuum removes any li(|uid, the medium through which the bacteria is able to move. As to the bacteria itself, the vacuum nec- essarily causes all the moisture to evaporate off its surface. However, this does not necessarily mean that all moisture must be given up from the internal portions of the bac- teria. As a matter of fact, bacteria have been left in the microscope under the vacuum for extended periods of time before visible DIFFRACTION PATTERN OF ZINC OXIDE OBTAINED WITH THE ELECTRON DIF- FRACTION CAMERA ATTACHMENT OF THE MICROSCOPE. changes occurred. This rule is not absolute for in some cases changes have been noted in very short times. As to the matter of the electron beam producing a change, it is gen- erall.v believed that the electrons bombarding the nuclei are fatal to the bacteria. Just recently a paper has been published abroad claiming that bacteria have survived photo- graphing in the microscope. Some work has been done on the application of heavy-metal germi- cides which react selectively with certain components of protoplasm but as yet no systematic study of the factors involved has been car- ried on and results so far obtained show definite promise of an inter- esting field of studv. At first, it was believed that the electron microscope would not be too successful in the study of solid surfaces. The metallurgist was in great need of an instrument of higher resolution than the light microscope for broadening h i s study of the surface of metals. There was soon developed the now famous replica method for adapting this instrument to special work. The original method consisted of preparing the surface to be studied, then evaporating a thin coat of silver onto this surface in a vac- uum. This silver film was then stripped from the surface and a thin collodion film applied to it. The silver film was then dissolved from the collodion film with acid, leaving (continued on page 30)