CHAPTER VIII. 



POLARIZED LIGHT THE SIMPLE POLARIZING MICRO- 

 SCOPECRYSTALS UNDER THE MICROSCOPE. 



Few chemists realize the value of employing polarized light 

 in connection with the microscopic examination of material 

 to be analyzed, and few appear to appreciate the great saving 

 of time, labor and reagents that such an application generally 

 affords. Even a cursory examination with the most simple 

 polarization devices is not to be ignored. 



In the microscopic study of material of unknown composition, 

 the first step of the chemist should be to subject it to polarized 

 light 



But in order that the polarization microscope may be employed 

 intelligently in the analysis of inorganic and organic materials, 

 it is essential that certain fundamental concepts of optics and 

 of crystallography be recalled; otherwise the phenomena ob- 

 served may not be properly interpreted. 



In ordinary light the ether vibrations are in all possible 

 azimuths and the path of vibration of any single ether particle 

 is constantly changing. Light so changed that the ether vibra- 

 tions take place in a direction parallel to a single plane is known 

 as polarized light. Polarized light may be either white light or 

 monochromatic light. 



All transparent (and translucent) bodies behave with respect 

 to light waves in one of two ways: (i) they are optically homo- 

 geneous and therefore have no effect upon a beam of light sent 

 through them, no matter what the direction may be ; such bodies 

 are called isotropic and exhibit but a single index of refraction; 

 ether waves proceeding from any point are spherical; (2) they 

 are not optically homogeneous but transmit light waves with 

 different velocities in different directions; in this case they are 

 called ceolotropic or anisotropic; ether waves proceeding from a 



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