IQ4 ELEMENTARY CHEMICAL MICROSCOPY 



doubly refracting crystal, the greatest index will be found in the 

 direction through the crystal of the lowest rate of light trans- 

 mission and the smallest index in the direction of the highest 

 rate of light transmission. In other words, different values for 

 the index of refraction will be obtained according to the position 

 in which the crystals lie upon the stage of the microscope. 



Crystals belonging to the tetragonal and hexagonal systems 

 (uniaxial crystals) possess two indices. Crystals belonging to 

 the orthorhombic, monoclinic, and triclinic systems (biaxial 

 crystals) have three indices. 



In uniaxial crystals one value corresponds to that given by the 

 ordinary ray and the other to that given by the extraordinary 

 ray. The first value is found in that direction through the crys- 

 tal where the light vibrations are transmitted transverse to the 

 vertical crystallographic (and in this case optical) axis and is 

 designated by the Greek letter o>; the second value is observed 

 when light is transmitted through the crystal parallel to the verti- 

 cal axis. This index is designated by the Greek letter e. The 

 double refraction of uniaxial crystals is said to be strong when o> 

 is greater than c, and weak when the reverse is found. When the 

 refractive index co is greater than e, the crystal is said to be opti- 

 cally negative and when less than e, optically positive. Some 

 crystallographers prefer to designate the two refractive indices 

 by the letters a and 7. In this case 7 a expresses the strength 

 of double refraction and when a is greater than 7 the crystal is 

 optically negative. 1 



In biaxial crystals three different values for the rate of light 

 transmission can be found, or in other words biaxial crystals have 

 three axes of elasticity or directions of vibration; the axis of 

 maximum rate of vibration transmission is commonly designated 

 by the German letter a ; that of the minimum vibration by c and 

 the intermediate axis by b. Since there are three axes of elas- 

 ticity, three different values for the index of refraction may be 

 obtained, the smallest value a in the direction of the maximum 



1 In order to be sure of the values for co and e, a number of different crystals 

 should be tried out. co will be constant in all of them, e will differ slightly accord- 

 ing to the position of the crystals. 



