ILLUMINANTS 169 



differences in optical path the darkfield and idtramicroscope are pref- 

 erable. A fragment of glass with a refractive index of 1.52 mounted 

 in balsam with an index of about 1.54 is shown in Fig. lYA. With 

 brightfield (A) the path difference is so small that the fragment is nearly 

 invisible and onl}' the reflections from the longer slanting surfaces are 

 visible with darkfield (B). With the phase microscope considerable 

 detail is seen clearly with bright (T), B— dark (D), and A— dark (E) 

 contrasts. Note the difference between dark-contrast phase micros- 

 copy {D, E) and darkfield (B) microscopy; these methods are com- 

 plementary, one being appropriate when the other is not. (3ptical 

 staining is often helpful in combination with phase microscopy (see 

 Section 3.5). An annulus of greater aperture than that of the objective 

 gives fairly good darkfield illumination (16-mm objective and 4- or 

 1.8-mm annulus) for comparison and is helpful for locating a transparent 

 specimen. 



The phase microscope promises to be more useful for internal speci- 

 men detail, and interference microscopy for the study of variation on 

 the surface of the specimen. Both methods are too recent for other 

 than broad generalizations. They may be used together or indiA'idually, 

 depending on the object of the investigation and the nature of the 

 specimen. 



Anisotropic materials are revealed to advantage with the polarizing 

 microscope. Two adjacent colorless, transparent, isotropic regions of 

 small difference in path can be discovered only with the phase micro- 

 scope. Materials that change the phase relations of the light passing 

 through them may or may not be observed to advantage with the aid of 

 polarized light in the phase microscope, as will be shown in Section I of 

 Chapter \1. Ultimately, when the theory of phase and polarization 

 microscopy is developed, prediction of the more useful method will 

 become possible. The occasional gains justify trying the combination. 



Rarely is a specimen found for which one, and only one, microscope 

 method is entirely adequate. Usually the image formed is a result of 

 the different properties of the specimen acting together on the radiant 

 energy used for probing its content, and each method of microscopy 

 will reveal additional information. The ideal microscope should be 

 fitted for all methods. Since this is not practicable at the present time, 

 the appropriate methods must be chosen with respect to the specimen 

 and the objective of the investigation (Richards, 1950). 



2. ILLUMINANTS 



The most useful source of light is the research type of microscope 

 illuminator with a focusable lens system, iris diaphragm, and filter 



