THE DETERMINATION OF REFRACTIVE INDEX 195 



axis a, the greatest value 7 in the direction of the axis c and an 

 intermediate value in the direction of the b axis. The double 

 refraction of the crystal will be strong or weak according to how 

 much greater 7 is than a. To determine whether a biaxial crys- 

 tal is optically positive or negative requires other data than 

 refractive indices. 



In uniaxial crystals the determination of which index is o> 

 and which e is comparatively simple since e coincides with 

 the crystallographic c axis; but in the case of biaxial crystals it 

 is seldom that a chemist possesses either the knowledge or a 

 microscope sufficiently well equipped to definitely locate the 

 different axes of elasticity, since their directions are indicated by 

 neither the crystallographic nor the optical axes. For this 

 reason it is wiser for the chemist-analyst to follow the methods of 

 Kley, 1 Bolland 2 and others, and record values as obtained in the 

 method given below. 



Swing the polarizer in place, having first removed all condens- 

 ing lenses. Place upon the stage an object slide carrying the 

 crystals or crystal fragments to be examined immersed in a 

 liquid of known refractive index and covered with a tiny thin 

 cover glass. Place the analyzer over the eyepiece (or slide it 

 into the tube if an instrument of this type is used) and set the 

 graduated circles of both prisms at zero so that their planes of 

 vibration are crossed. Turn the stage of the microscope until 

 the crystal selected for observation extinguishes; remove the 

 analyzer. Ascertain by raising the objective whether the index 

 of the crystal is greater or less than the liquid; check results by 

 oblique light by placing the finger part way across the opening 

 of the polarizer. Substitute one liquid after another until the 

 refractive index of the crystal is ascertained, being very careful 

 not to alter the position of the crystal. If the crystal is moved 

 replace the analyzer and readjust the crystal to the position of 

 extinction. Read the position of the crystal as indicated on the 

 circumference of the stage and rotate the stage so as to turn the 

 crystal exactly 90 degrees to its position of extinction and proceed 



1 Kley, Zeit. anal. Chem., 43 (1904), 160. 



2 Bolland, Monats., 29 (1908), 991; 31 (1910), 387. 



