348 OSCAR W. RICHARDS 



2. Nature of Specimen Determines Choice of Microscope and 



Method 



Knowledge of how the specimen may affect the probing radiation 

 used for the formation of the image leads to the choice of the appro- 

 priate microscope and the best method for the most efficient examina- 

 tion of the specimen. Image formation may depend on one or a com- 

 bination of the following properties of the specimen: (a) Transmis- 

 sion: uniform or selective by wavelength. (6) Reflection, (c) 

 Optical path effects: refraction, retardation, (d) Diffraction, (e) 

 Scattering. (/) Polarizing, rotation of the plane of polarized light; 

 pleochroism, {g) Fluorescence and phosphorescence. 



Usually a specimen is partially transparent, or may be cut or 

 ground thin enough to transmit enough radiation for its examination. 

 Few specimens are uniformly transparent to all wavelengths. Most 

 specimens absorb some wavelengths of radiation more completely 

 than others and when this selective absorption is in the region to 

 which the eye is sensitive (380 to 740 mju) color results. Colored and 

 gray images are visible and such a specimen would be examined with 

 the brightfield microscope (Sect. B). Wlien the selective absorption 

 occurs for shorter wavelength radiation the idtraviolet microscope 

 should be used and when of longer wavelength the infrared micro- 

 scope is the instrument of choice (Sect. C). When the absorption is 

 weak, phase microscopy will be helpful (Sect. I). Opaque and nearly 

 opaque specimens can be examined by reflected light using vertical 

 and epi methods of microscope illumination (Sect. D). The hi- 

 ohjedive microscope (Sect. D2) reveals three dimensional form at 

 lower magnifications and for the higher magnifications the brightfield 

 compound microscope is used. 



Quite transparent objects usually contain regions of different re- 

 fractive index and size, w^hich alter the direction and speed of light 

 passing through them. The product of the thickness (t) of the region 

 times its refractive index (n) is the optical path, which is sometimes 

 expressed in w^avelengths (nt/X). When these path differences are 

 small, phase microscopy (Sect. I) will reveal otherwise invisible detail, 

 but when the optical path differences are large the darlfield methods 

 (Sect. E) will be more useful to the microscopist. Surface detail of 

 some specimens and nonuniformities of optical path in transparent 

 specimens may be observed with the interference microscope 

 (Sect. H). Diffraction contributes to most image formation and a 



