specialized Microscopy 99 



A polarizing (prism) in the fork substage instead of a condenser, or a 

 polaroid disc in the condenser slot polarizes the light so it vibrates in 

 one plane only. Part of the light (ordinary ray) is reflected to the side of 

 the prism and does not illuminate the object; the other part (extraor- 

 dinary ray) continues straight through the prism to emerge as polarized 

 light. (There obviously is a loss of light, so plenty of it must be used.) 

 If the polarizer is a prism, it usually can be rotated 360° and the amount 

 of rotation in the field checked. 



In or above the ocular is fitted another polarizing prism, the analyzer, 

 whose vibration direction is set at 90° to that of the polarizer. The 

 extraordinary ray from the polarizer becomes the ordinary ray in the 

 analyzer and is reflected out of the field, unless an anisotropic sub- 

 stance (doubly refracting when placed between the analyzer and polar- 

 izer) rotates the plane of polarization and interferes with the path of 

 light. Such a substance divides the light from the polarizer into two 

 beams, one of which passes through the analyzer, making the object 

 appear to glow against a dark background. Isotropic substances (singly 

 refracting) do not polarize and therefore do not divide the beam of light 

 and do not glow. 



SOME USES OF POLARIZING MICROSCOPY 



1. Determines whether an object is isotropic or anisotropic (if it rotates the 

 plane of polarization). 



2. Can be used for differences in physical properties in different directions; 

 study of mitotic spindles. 



3. Can be used on fresh unfixed material. 



4. Reveals molecular orientation of structures, chemical constitution, chem- 

 ical and physical intervention in the cell, pressures or tensions; all can 

 produce anisotropic effects. 



5. Can be used on natural and artificial fibers, cellulose fibrils, lamellar 

 plasm differentiation, pseudopodia, spindles and asters, nerve fibers, 

 muscle fibers, chromosomes, chemical and mineral crystals, crystallized 

 hormones and vitamins, dust counts, starch grains, horn, claw and bone 

 sections. 



6. Can be used for determination of refractive indices. 



X-Ray Diffraction 



All forms of matter scatter X-rays and form diffraction patterns. From 

 these patterns information concerning the materials can be ascertained; 

 crystalline proteins show an X-ray diffraction of great complexity. 



