IiNDLSTKlAL RESEARCH 



often the specimen is surrounded by a 

 mounting medium, such as balsam, placed 

 between the slide and cover-glass. The re- 

 fractive index of the mounting medium often 

 determines what is visible in a specimen. 

 For example, if the external features of a 

 transparent subject are to be visible, the 

 refractive index of the mounting medium 

 must differ from that of the subject. If, on 

 the other hand, the internal structure is to 

 be studied, confusing external features may- 

 be made invisible by placing the specimen in 

 a medium having a matching index of refrac- 

 tion. Figure 5 shows an example; e.g., a 

 sodium chloride crystal which was mounted 

 in a matching medium to reveal brine inclu- 

 sions. 



Because different levels are visible in 

 transparent or semitransparent specimens, 

 it becomes necessary to make thin specimens 

 so as to confine observation to a thickness 

 within the range of the depth of focus of the 

 objective. Otherwise, out-of-focus structure 

 detracts from the image quality. Small par- 

 ticles and liquids are easily made into thin 

 preparations, but reducing the thickness of 

 large rigid specimens poses somewhat of a 

 problem. Thin sections of materials of the 

 proper consistency to be cut can be pro- 

 duced with a microtome, or sections of 

 minerals and ceramics can be made by modi- 

 fied metallographic techniques (25). The 

 latter involves cutting slices of the sample as 

 thin as possible with a cutoff wheel, attach- 

 ing the slice to a microscope slide and thin- 

 ning it further by grinding and polishing the 

 exposed side. If the section is mounted in a 

 medium with a matching refractive index, 

 distracting surface irregularities are invisible. 

 Only the internal structure is visible. When 

 transmitted and vertical illumination can be 

 employed simultaneously, a suitably highly 

 polished and/or etched surface is not covered 

 with a mounting medium or cover-glass. 



Thin sectioning with a microtome has 

 been well established in microscopical science 



Fig. 5. A .salt crystal mounted in a matching 

 refractive index medium to reveal brine inclu.sions. 

 130X. 



for many years; however, renewed interest 

 in the development of refined techniques for 

 the purpose of electron microscopy has 

 pointed the way to extending those for light 

 microscopy. Especially the embedding of 

 materials in plastics has been found appli- 

 cable to microtomy for light microscopy. The 

 glass knife, also used routinely by man\^ in 

 electron microscopy, has been found useful 

 in cutting thicker sections for light micros- 

 copy of materials embedded in these plastics. 

 Sections of paper and textiles as thin as 3 

 microns have been cut routinely by these 

 techniciues. Figure 6 shows a typical 3-mi- 

 cron section of paper. Embedding procedures 

 can be modified according to the nature of 

 the material. In this respect, often it is 

 possible to become less inhibited as to the 

 emfjedding procedures than in the case of 

 biological specimens. Many materials can 

 withstand higher temperatures or solvent 

 and chemical actions which are ordinarily 

 avoided in embedding procedures. For ex- 

 ample, the procedure used in the authors' 

 laboratory for embedding paper commences 

 by soaking it in alcohol. In fact, when the 

 paper was first soaked in water, a swelling 



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