POLARIZED LIGHT 



278 



POLAROGRAPHIC TECHNIQUE 



or muscle will appear blue in one diag- 

 onal position and yellow in the diagonal 

 perpendicular thereto; this is because 

 these j&bers manifest birefringence 

 which is positive with respect to the 

 fiber axis. A nerve fiber shows the 

 same colors in its myelin sheath except 

 that the diagonal positions in which it 

 shows these colors are reversed from 

 those of the above case; this is because 

 the myelin sheath manifests birefrin- 

 gence which is negative with respect to 

 the fiber axis. 



The birefringence of most biological 

 objects is due to regularity of structure 

 of components considerably smaller 

 than the wavelength of light. To get 

 at the nature of these components, one 

 studies the relation of the birefringence 

 to the refractive index of the medium 

 in which the object is immersed, using 

 consecutively a number of media (us- 

 ually organic solvents) of varying re- 

 fractive index. Application of Wie- 

 ner's theory then makes it possible to 

 deduce the orientation of the submicro- 

 scopic particles as well as their internal 

 regularity of structure, refractive in- 

 dices and approximate partial volumes. 

 Electron microscope observations 

 have confirmed many of the deductions 

 based on the polarization optical anal- 

 ysis of tissue ultrastructure. This 

 method will continue to be of impor- 

 tance biologically despite the great 

 possibilities of the electron microscopy, 

 for the polarized light method is appli- 

 cable to tissues in the fresh state. See 

 Schmidt,W.J.,DieDoppelbrechungvon 

 Karyoplasma, Zytoplasma und Meta- 

 plasma, Berlin Geb. Borntrager, 1937. 

 Frey-Wyssling, A., Submicroscopic 

 morphology of protoplasm and its 

 derivatives, Elsevier Publishing Co., 

 Inc., 1948. Schmitt, F. O., The ultra- 

 structure of protoplasmic constituents. 

 Physiol. Rev., 1939, 19, 270. Schmitt, 

 F. O., Tissue structure: polarization 

 optical analysis. In Glasser's Medical 

 Physics, Vol. II, 1950, p. 1128. 

 Bennett, H. S., The microscopical in- 

 vestigation of biological materials with 

 polarized light. In McClung's Micro- 

 scopical Technique, in press. 



Polarized Light is said to be better than 

 Marchi and Sudan III methods for 

 study of myelin degeneration of periph- 

 eral nerves (Prickett, C. O. and Stevens, 

 C, Am. J. Path., 1939, 15, 241-250). 

 Used in study of mitochondria and 

 Golgi apparatus (Monn6, L., Pro to - 

 plasma, 1939, 32, 184-192). 



Polarizing Microscope. The polarizing mi- 

 croscope in its simplest form is a con- 

 ventional microscope used with plane- 

 polarized light for illumination and with 



a polarizing ocular (called the analyzer) 

 to detect the presence of birefringence. 

 Or it may be thought of as a polariscope 

 equipped with magnifying lenses. This 

 instrument has been borrowed from 

 the minerologists, who employ it in 

 the study of crystalline materials. Its 

 purpose in histology is to seek sub- 

 stances characterized by their ability 

 to rotate the plane of polarized light 

 (Schmitt, 1939). 



A Nicol prism, or a disk of Polaroid, 

 is placed below the condenser. Ordi- 

 nary light is thereby polarized, but goes 

 on to form an image, exactly as in the 

 bright-field microscope. With an ana- 

 lyzer in the ocular, oriented so that its 

 polarizing direction is parallel to that 

 of the polarizer below, one sees the 

 regular image. With a 90° rotation of 

 the analyzer, however, the field be- 

 comes completely dark, except in those 

 places on the slide where birefringent 

 crystals or crystalloidal materials are 

 found. Since this image in general will 

 not be very bright it is best to observe 

 with the dark-adapted eye and to have 

 a very strong source of light for so much 

 light is lost. Starch grains, cholesterol 

 droplets, certain salt crystals, any or- 

 ganic structures with preferentially 

 oriented molecular aggregates, such as 

 nerve fibers (Prickett, C. O. and 

 Stevens, C, Am. J. Path., 1939, 15, 

 241-250), striated muscle, bone (Klee- 

 men 1945) plant cell walls, etc. stand out 

 well (Johnson, B. K., Endeavour, 1948, 

 7, 57-65). With a more elaborate 

 polarizing microscope one may make 

 quantitative measurements leading to 

 the identification of these materials 

 and to certain conclusions regarding 

 the fine structure of organic forms. 

 See Polarization Optical Methods. 

 Polarographic Technique — Written by 

 Christopher Carruthers, Division of 

 Cancer Research, Washington Uni- 

 versity, St. Louis, 10, Mo. October 5, 

 1951 — The polarographic method of 

 analysis is ideally suited for the quanti- 

 tative and qualitative measurement of 

 minute amounts of reducible sub- 

 stances. It is especially suitable for 

 concentrations of 10"'^ to 10"^ molar, 

 and since the analysis can be performed 

 with a very small volume of solution, 

 as little as a drop to a few tenths of a 

 cc, traces of reducible substances can 

 be detected. This method was dis- 

 covered by Professor J. Heyrovsky of 

 Charles University, Prague (Rev. trav. 

 chim., 1925, 44, 488-498). It is based 

 upon the reducibility, or oxidizability, 

 of a substance. For example, when 

 electrons are removed from the ferrous 

 ion, Fe*"*^, it is oxidized to the ferric 



