GE.N EH A 1. >I I ( KOSCOP Y 



Fig. 1. A photomicrograph of starch grains 

 under polarized light. The black cross exhibited by 

 each starch grain indicates that it has anisotropic 

 crystals radiating from the center. Such a struc- 

 ture is called a spherulite. 



latter is employed to enhance contrast and 

 to compare orientation among the grains of 

 polished and etched specimens. It has been 

 found especially useful in locating corrosion 

 and other reaction products. 



Dark -Field Illumination. Dark-field 

 illumination is effective in obtaining contrast 

 and for observing the colors of structures. 

 Incident dark-field has been used very suc- 

 cessfully in the detection of small quantities 

 of materials lying on the surface of a sub- 

 strate, particularly when sparsely dis- 

 tributed. The dark-field condenser is con- 

 structed to prevent any light from entering 

 the objective but that which is scattered by 

 the specimen. The specimen is illuminated 

 by a hollow cone of light of too large an angle 

 to permit the direct beam to enter the ob- 

 jective. The dark-field image is characterized 

 by a dark background with more or less 

 bright structures revealed in the subject. 

 There is an intermediate case of hollow 

 conical illumination in which a portion of 

 the undeviated beam is allowed to enter the 

 objective, in order to enhance contrast. This 



is discussed in further detail later in this 

 article. 



Improvement of Contrast by Speci- 

 men Preparation. As mentioned previ- 

 ously, contrast can be improved by speci- 

 men-preparation methods. Staining, etching, 

 metal shadowing, selecting appropriate 

 mounting media, and replicating are some of 

 the sample manipulations used to increase 

 the contrast of the preparation. 



Staining (6) has been used in biological 

 preparation for years, and it is feasible to 

 extend staining techniques to industrial 

 samples, even metallurgical preparations. 

 Some techniques are already developed, but 

 frequently it is required that a technique be 

 devised to satisfy a specific need. In view of 

 the numerous stains now available and also 

 the known reactions which yield colored 

 reaction products, usually it is not difficult 

 to select or develop a suitable staining pro- 

 cedure. Stains are employed to dye selec- 

 tively certain structures of interest. They 

 may be merely selective in the sense that 

 they reveal structure without being recog- 

 nizably indicative of chemical composition, 

 or on the other hand, they may be strictly 

 specific for a chemically reactive ion or 

 group. In the latter case it is obvious that 

 much is to be gained in relating the dis- 

 tribution of the chemical substance which 

 reacts with the stain to the over-all structure 

 of the specimen. 



Thin sections sliced with a microtome (6) 

 are quite appropriate for staining. Even in 

 the unstained condition, microtome-pro- 

 duced thin sections are inherently more 

 contrasty subjects with less confused struc- 

 ture than is the larger sample from which 

 they were cut and thus permit the observa- 

 tion of the internal morphology of the 

 sample. The biologist has pioneered in the 

 field of microtomy, but again this is an 

 effective technique for the industrial worker 

 who need not be limited to employing the 

 techniques of the biological micromotist. 

 Because, in nonbiological problems, it is not 



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