XI. WHEN TO USE SPECIAL MICROSCOPES 349 



pure diffraction image is formed when the specimen is smaller than 

 the limit of resolution of the objective used. If the illumination is 

 sufficiently intense the bright central part of the diffraction disc (Fig. 1) 

 may be seen with darkfield or phase microscopes (Sect. El, Sect. I) 

 and some knowl^tlge gained of the average size and distribution of the 

 particles, even though the size and shape of the individual particle may 

 not be measurable. 



\Vlien the specimen orients the vibration direction of the light 

 passing through it, or rotates the plane of polarized light passing 

 through it, the polarizing microscope should be used (Sect. F). 



Some specimens will absorb radiation of one wavelength and re- 

 emit the energy at another, usually longer, wavelength. This is 

 called fluorescence. Also, objects may be treated with a fluorescent 

 chemical, a procedure analogous to staining, and examined hy fluores- 

 cence microscopy (Sect. G). Phosphorescing specimens continue to 

 emit light for a short time after the exciting energy is stopped and a 

 microscope has been devised for work with such specimens (Sect. G). 



Rarely is the image seen in the microscope due to a single one of 

 the above causes. Even so, when one image type predominates, the 

 kind of microscope indicated becomes the instrument of choice. 

 WTien the specimen gives several types of image, it may be necessary 

 to use several or all of the microscopes and methods to fully compre- 

 hend the specimen. In general these methods are complementary 

 and the objective of the investigation will assist in the choice of in- 

 striunent. Since it is impractical to cross-index the interrelations of 

 all these methods in this section, it is suggested that the investigator 

 read the entire chapter for a broad over-all concept of the possibilities 

 of microscopy before selecting the procedure to be used in a given in- 

 vestigation. 



B. COLORED SPECIMENS— BRIGHTFIELD MICROSCOPY 



Specimens selectively absorbing or transmitting wavelengths of 

 light to which the eye is sensitive have color. The image from such 

 a specimen is likewise colored and the color aids in recognizing the 

 fine structural detail. Selective staining of specimens with dyes may 

 differentiate and make possible the observation of detail that would 

 otherwise not be visible. The ordinary brightfield microscope is used 

 for the examination of colored objects. 



Achromatic objectives are corrected for chromatic aberrations 



