MICEOSCOPIC IDE^STTIFICATION OF INOEGAXIC SALTS. 7 



clined illumination may be used to determine whether the substance 

 has an index higher or lower than that of the oil. This determined, 

 the substance is mounted in another oil of appropriately higher or 

 lower index, as the case may be, and the operation repeated. By 

 this method of trial and error, an oil is found in which the given 

 substance disappears. The refractive index of the substance is then 

 the same as that of the oil. A little practice enables one to estimate 

 quite closely the index of a substance observed in an oil of a definite 

 but different index, and this estimation very materially reduces the 

 number of trials that must be made before the correct oil is found. 



The preceding description applies directly to the measurement of 

 the refractive index of an isotropic substance. In the measurement 

 of the indices of anisotropic substances, the problem is complicated 

 by the fact that the index of the substance varies as the direction of 

 the vibration of the light varies. With anisotropic substances, it is 

 therefore necessary to ascertain carefully the optical orientation of all 

 particles upon which the measurements are carried out. On a given 

 substance in the proper orientation one index may be determined 

 directly. A second index is in the same plane and consequently may 

 be determined by a change in the orientation of the substance repre- 

 sented by simple rotation of the stage of the microscope. The third 

 index must be measured upon the substance in another plane repre- 

 sented by a tilting of the grain. This tilting may be accomplished 

 by a imiversal stage attachment on the microscope or by moving the 

 cover glass, as, for example, with the point of a pencil. In the 

 measurement of indices of anisotropic substances, regard to optical 

 orientation is more often than not a considerable aid in identification. 



Whether a given crystal is isotropic or anisotropic — that is, whether 

 it is isometric or belongs to one of the other crystalline systems — is 

 readily determined by crossing the nicol prisms of the microscope. 

 Isotropic substances transmit no light under these conditions ; whereas 

 anisotropic substances, being doubly refractive, do transmit light and 

 are readily visible. In making this test care must be taken that an 

 anisotropic uniaxial substance is not so oriented that the line of 

 vision is parallel to the optic axis of the substance and that no par- 

 ticles are observed only at their point of extinction. Tilting the grain 

 and revolving the stage of the microscope will eliminate both of these 

 possibilities of error. If the substance is known to be biaxial, simple 

 revolution of the stage is sufficient. 



Uniaxiality or biaxiality is established by converting the micro- 

 scope into a conoscope by crossing the nicols and removing the ocular 

 or by inserting the Bertrand lens and observing the interference figure 

 of the particle in question. Uniaxial substances show a cross, if lying 

 on a plane perpendicular to the optic axis. As the particle is tilted 

 more and more out of this position the center of the cross finally is out 



