106 MICROSCOPY 



are optically isotropic. Other materials, notably crystals and colloidal 

 aggregates, show a different behavior depending on the direction in 

 which the light beam passes through them. Such materials exhibit one 

 or more of several forms of optical anistropy. 



Most optically anistropic materials differ in refractive index in differ- 

 ent directions, which means that the polarized light passes through the 

 material more rapidly in one direction than in others. Such materials 

 are doubly refractive or birefringent. The effect would be apparent in a 

 material such as that shown in Fig. 8-13. Rods of one substance, with 

 a characteristic refractive index, are aligned parallel to each other in a 

 medium of different refractive index. The polarized light is propagated 

 in the direction parallel to the fibers with a velocity different from the 

 velocity across the fibers. If this birefringence is a property of the material 

 itself and not of the medium in which it is mounted under the micro- 

 scope, the material exhibits intrinsic birefringence. A similar effect occurs 

 when particles with one refractive index are oriented in a medium of 

 different refractive index, but this effect— form birefringence— might dis- 

 appear if the medium is changed. Components of protoplasm seem to 

 show both these effects under certain conditions. 



Dichroism is dependent upon differences in absorption of polarized 

 light passing through in different directions. Certain colors of light 

 are absorbed when the material is oriented in a particular way, while 

 another pattern of absorption appears if the material is rotated 90°. 

 Thus the material will change color as it is rotated. 



The analysis of materials with the polarizing microscope depends upon 

 changes occurring when the polarizer, the material, or the analyzer are 

 rotated. The observer may see alternations of light and dark fields, 

 changes in color, or a series of color fringes reminiscent of spectra. 

 Interpretations require experience, and only relatively few biologists 

 have used the polarizing microscope. There is a body of literature 

 including studies on protoplasm, however, and some of our knowledge 

 of the orientation of materials in protoplasm has resulted from such 

 studies. 



Another fairly simple effect of some materials on the beam of polarized 

 light is a rotation of the plane of polarization. If the polarizer of the 

 microscope is set so that the light is vibrating in the "twelve o'clock to 

 six o'clock" plane, the material on the stage will rotate this plane to the 

 right or left by an amount which depends on the nature of the material 

 and its thickness. The degree of rotation can be determined by rotating 

 the analyzer until the field looks bright. Ordinarily this effect is small. 



