OPTICAL THKORY OF LIGHT MICROSCOPE 



ence of the diffracted and iindiffracted por- 

 tions simulates that of a specimen having 

 density variations. 



For a more thorough understanding of 

 phase contrast the reader is referred to 

 "Phase Microscopy", by Bennett, Jupnik, 

 Osterberg and Richards. 



Polarized Light is another method of 

 bringing out visible structure in transparent 

 materials. Light energy is transmitted by 

 transverse waves, generally vibrating in any 

 direction at right angles to the direction of 

 the light ray, but it is possible by means of 

 devices known as "polarizers" to restrict the 

 vibrations to a single plane. If two such 

 polarizers are inserted in the light beam in 

 such a manner that the second one trans- 

 mits in a plane at right angles to the first, 

 the light will be extinguished. Such an ar- 

 rangement is called "crossed polarizers". The 

 lower element is called a "polarizer"; the 

 upper one the "analyzer". In practice, the 

 polarizer is located beneath the microscope 

 condenser j and the analyzer just above the 

 objective. If now, in between these crossed 

 polarizers we insert an object which is crys- 



talline in nature, it will appear bright against 

 the dark background caused by the crossed 

 polarizers. Furthermore, if the crystal is ro- 

 tated about the optical axis of the micro- 

 scope, it will alternately brighten and darken 

 every 90°, usually with attendant strikingly 

 beautiful changes of color. The reason for 

 this is that crystalline materials have differ- 

 ent properties in different directions, and as 

 a conseciuence they alter the state of polari- 

 zation of the light, and thereby effectively 

 "uncross" the polarizers. 



Polarized light has long been a mainstay 

 in the study of crystals, minerals, chemicals, 

 and fibers. Special microscopes called "pet- 

 rographic" or "chemical microscopes" are 

 used for any serious work in these fields. 

 These are equipped with accessories for mak- 

 ing quantitative measurements of the polar- 

 izing characteristics of the material under 

 study. Fig. 8 shows a typical Petrographic 

 Microscope. It is equipped with a circular 

 rotating stage, divided in degrees, with a 

 vernier permitting reading of crystal orien- 

 tation of 0.1°. The objectives are mounted 

 in centerable mounts, since it is necessary 



COARSE FOCUS 



8ERTRAND SWING "OUT 

 AND IRIS 



ANALYZER SWING "OUT 

 AND ROTATION 



MECHANICAL STAGE 



FINE FOCUS- 



CONDENSER SWING -OUT 



CONDENSER FOCUS 



BERTRANO FOCUSING 



ACCESSORY SLOT 



CENTERABLE OBJECTIVE 



GRADUATED ROTATING 

 STAGE 



CIRCULAR POLARIZER 



POLARIZER ROTATION 



k 



Fig. 8. A Typical Petrographic Microscope. This specialized microscope for use in the study of 

 crystalline materials differs in many respects from the conventional microscope, as indicated by the 

 many special control knobs in the figure. 



453 



