XI. WHEN TO USE SPECIAL MICROSCOPES 365 



When the Hght is emitted for a short period after the exciting 

 radiation is turned off the material is called phosphorescent; a micro- 

 scope for the examination of such specimens has been designed by 

 Harvey and Chase {34)- 



H. SURFACE IRREGULARITIES AND OPTICAL 

 NONUNIFORMITY OF SPECIMENS— INTERFERENCE 



MICROSCOPY 



Two plane-parallel pieces of glass in perfect contact will show only 

 a uniform color when monochromatic light is passed through them, 

 but, when the surfaces are not in uniform contact, interference 

 fringes are seen around any imperfect region ; when the two surfaces 

 are inclined to each other interference bands become visible. The 

 fringes are due to the combination of the light waves from the multi- 

 ple reflections {S8). Brighter fringes are seen when the two reflecting 

 siu'faces are covered with a partially transparent film of silver, plat- 

 inum, or other good reflector. Interference methods are useful for 

 the measurement of surface irregularities, small displacements, and 

 inhomogeneities in transparent material and are generally used for 

 precise measurements in the testing of optical elements and with gage 

 blocks. A special interference microscope has been made for the 

 examination of metal and other highly reflecting surfaces (68). 

 Microscope fine adjustments are calibrated with an interferometer 

 {1; cf. Sect. L3). 



A multiple beam method has been used by Tolansky (35) for 

 the examination of crystal surfaces and extended to biological sur- 

 faces by Greenham (36), who pictures the cuticular surface of an 

 apple leaf. Merton (37) reported that it was unnecessary to use 

 expensive optical flats, since selected pieces of glass or plastic films 

 could be partially platinized and used. The specimen is mounted 

 between two pieces of platinized glass, or plastic and glass, and ex- 

 amined with a brightfield microscope using monochromatic light. 

 The distribution and shape of the interference fringes indicate the 

 structure of the specimen. The optical path is 2 tn cos 6 where t is 

 the distance between the plates, n is the refractive index, and 6 is 

 the angle made by the rays with the normal. Thickness is obtained 

 by counting the number of fringes; N fringes denote a thickness of 

 NX/2. Broader fringes were obtained when the lower side of the sup- 

 port plate was a ground surface that revealed considerable detail 

 within the transparent specimens. 



