XI. WHEN TO USE SPECIAL MICROSCOPES 361 



ored materials have been examined to show the penetration of the dye 

 and the nature of the parts of the specimen stained. The stops are 

 available as discs with various color combinations {62), with a metal 

 form to hold interchangeable colored discs and annuli {58), and as a 

 complete substage condenser unit {68). The results are spectacular, 

 but often do not show as much detail as darkfield illumination. 

 Royer and associates have recently evaluated the method {25). 



Optical staining may also be obtained by utilizing the differences 

 in dispersion of light by regions of the specimen having different re- 

 fractive indices for colored light of different wavelengths. The speci- 

 men is mounted in a medium of proper refractive index and illumi- 

 nated with white light by means of a darkfield condenser. Regions 

 of different index, or bending power, are then seen in different colors. 

 Differentiation within tissues and minerals is possible and the method 

 is useful for locating impurities in pulverized material. The method 

 is new, although based on known principles, and promises greater 

 utility with low and medium than with the greatest magnifications 

 {26). Besides a darkfield condenser, media compatible with the 

 specimen must be available in a series with small differences in refrac- 

 tive index, or must be made by mixing as required to bring out the 

 color differences of the specimen. Both methods permit examination, 

 of many materials with little or no damage to the specimens. 



F. SPECIMENS THAT POLARIZE OR CHANGE 

 POLARIZED LIGHT 



When the direction of vibration of light changes or when the 

 speed of the light depends on its direction of vibration as it passes 

 through the specimen, polarization microscopy will be useful. Ordi- 

 nary light consists of electromagnetic vibrations in all planes at right 

 angles to the direction of its travel. Some substances polarize the 

 light that passes through them so that the emergent beam vibrates 

 in a single plane, or other geometrical form. 



Special optical equipment must be added to the ordinary micro- 

 scope in order to permit the measurement and analysis of the various 

 changes in light that may be produced by such an object. A polariz- 

 ing device, called a polarizer, is placed below the microscope con- 

 denser and another (the analyzer) is placed over the ocular, or within 

 the body tube of the microscope; at least one of them should be 

 provided with a scale for measuring angular rotation. The polariz- 



