4 BULLETIN 1108, U. S. DEPARTMEIvrT OF AGRICULTURE, 



All of the optical properties named may usually be observed and 

 measured where measurement is possible in about 15 minutes, with 

 no more material than can be held upon the point of a knife blade, 

 assuming of course that one has ready all the essential equipment for 

 the work and is not under the time-consuming necessity of making a 

 microscopical search for the material to be identified in a mass of other 

 material. In certain cases, however, the time of search can be mate- 

 rially reduced or even eliminated. For instance, if one has a mixture 

 of NaCl and KCl, the presence of either one but not both being 

 known, the mass can be mounted in an oil of the same refractive index 

 as that of the known substance, when the latter will be invisible or 

 practically so and the unknown will stand out with a considerable 

 degree of prominence, thus permitting ready ''spotting" and a sub- 

 sequent working down or up to its index with different oils. The 

 value of this point in the rapid detection of impurities in supposedly 

 pure substances is obvious. 



EXPLANATION OF SYMBOLS. 



For convenience in identification the substances have been tabu- 

 lated in five groups as follows: Isotropic, uniaxial positive, uniaxial 

 negative, biaxial positive, and biaxial negative substances. In the 

 isotropic group the refractive index is the only property to be ob- 

 served, and the arrangement of the substances is according to incieas- 

 ing values of the index. In the uniaxial positive and uniaxial nega- 

 tive groups there are two refractive indices to be measured, designated 

 CO and e, denoting respectively the ordinary and extraordinary rays of 

 light as observed in polarized light. The difference between these 

 two indices constitutes the birefringience of the substance. Other 

 properties, such as pleochroism, are occasionally observable, and the 

 crystal system should always be noted where possible. The arrange- 

 ment of the substances is according to ascending values of the w 

 index. 



In the biaxial positive and biaxial negative groups there are three 

 refractive indices to be measured — that is, the a, /S, and 7 indices — 

 indicating respectively the least, the intermediate, and the greatest 

 index of the substances. The differences between any two of these 

 are significant and are tabulated. The true optical angle is indicated 

 by 2V. The symbols 2E and 2H indicate the value of this axial 

 angle when measured in aii' and oil, respectively. Dispersion is tabu- 

 lated as greater or less for red, p, than for violet, u, light. In the 

 columns headed ''orientation," X, Y, and Z signify the directions of 

 vibration of the a, ^, and 7 rays. Ax.pl. is an abbreviation for plane 

 of the optic axes of the crystal. All the other symbols in this column 

 are either geometrical or simple crystallographic designations of crys- 



