"98 Special Procedures III (chap. 24) 



microscope and has revealed siibmicroscopic structures invisible until 

 the present time. It uses electrons as the illuminatin,^ beam, and focuses 

 the beam on the object by the use of magnets. As the beam passes 

 through the object under observation, some of the electrons are scat- 

 tered by the object causing shadows of the scattered electrons on photo- 

 graphic film. 



Electron beams must be handled in vacuum, therefore no ^vet or liv- 

 ing tissue ordinarily can be observed. Sections must be thin to prevent 

 appreciable electron absorption, because the image depends on the dif- 

 ferential scattering rather than absorption. 



Speciinen Supports 



The specimen is supported on a metal grid and is placed in the column 

 at the electron-gun side of the object-lens aperture. This mechanical 

 support is made of electron-opaque metal (copper, nickel or steel) per- 

 mitting observation of the specimen through holes in the grid. A mesh 

 of 200 holes per linear inch is satisfactory and provides holes small 

 enough to support a thin film yet offers a large enough area for observa- 

 tion. England prepares an excellent grid with a center marker and a 

 solid periphery (obtainable from Ernest Fullam, Schenectady, N.Y.). 



All specimens must be placed on some supporting film, supported in 

 turn on the grid. This film has to be transparent to the electron beam 

 but strong enough to not tear when bombarded by the beam. Various 

 materials have been used, but celloidin and formvar are favored. Car- 

 bon films are becoming popular, but are difficult to prepare {Watson, 

 1956; Barton, 1960). 



Preparation of Film 



Either celloidin, 1% in amyl acetate, or formvar, 0.2% in ethylene 

 dichloride (1,2 dichloroethane) is flowed on a clean dry microscope 

 slide, or the slide is dipped in the solution (slide may be previously 

 treated with a water repellant). Evaporate film on slide and when dry 

 float film off on surface of water. Place grids, convex side down, on film. 

 Lower a clean slide on top of grids and sweep the slide through the 

 water bringing out with it the grids lying between the film and the slide. 

 Allow to dry. Grids can be left on the slide luitil used. The film also 

 may be cast directly on the surface of the water, the grids placed on it 

 and removed as above. The former method, however, forms a tougher 

 film (Mellors, 19'>9). Form\ar makes a stronger and more temperature- 



