134 F. E. Wright — Index Ellipsoid ( Optical Indicatrix) 



tallinity. The determination of a few of the optical constants 

 of the crystal plate is, moreover, usually sufficient for the 

 identification of the mineral from which the plate has been cut ; 

 it is for this reason especially that the practical determination 

 of the optical constants of minerals in the thin section is of 

 fundamental importance in petrography. 



Experience in crystal optics has taught that a number of 

 different geometrical surfaces are useful in the solution of 

 problems on the propagation of light in crystals. Thus, (1) 

 the index ellipsoid, (2) the ovaloid of normal velocities, (3) the 

 Fresnel ellipsoid, (4) the inverse ovaloid of ray velocities and 

 the higher-order surfaces derived from these ellipsoids and 

 ovaloids, (5) the surface of normal velocities, (6) the index sur- 

 face, (7) the wave, or ray, surface, and (8) the reciprocal wave 

 surface are cited in treatises on crystal optics. For a given 

 set of conditions experience has taught further that a particular 

 one of the above surfaces is better adapted than any of the 

 others to express clearly the relations which obtain. This has 

 been, in fact, the incentive for the development of so many sur- 

 faces and for their detailed treatment in the text-books. They 

 aid in presenting the same subject from different view-points, 

 emphasis in one surface being placed on the wave-front nor- 

 mals, in another on the wave directions, and so forth. These 

 different modes of expression are essential and convenient for 

 the investigator who is working constantly at the subject, but 

 their great variety and yet close similarity renders their use 

 and application difficult for the microscopist, who readily for- 

 gets the nice distinctions between them, and is, as a result, 

 often confused and bewildered in his attempt to explain satis- 

 factorily, by their use, the optical phenomena he observes 

 daily in the study of thin sections of rocks. The phenomena 

 of interference in birefracting crystal plates are essentially 

 spacial phenomena, and their description and explanation 

 involve the concepts of tri-dimensional space. Now most 

 observers do not think readily in terms of space, and as a result 

 consider crystal optics an exceedingly complex subject, diffi- 

 cult to master but only too easy to forget unless practised con- 

 stantly. It is important, therefore, to present the subject as 

 applied to petrographic-mibroscopic work in a clear, simple 

 manner, and to burden the mind of the observer with as few 

 details as possible. 



It has long been felt that, for the explanation of the par- 

 ticular group of phenomena which the petrographer encounters 

 in work with thin rock-sections, a single one of the above sur- 

 faces would be adequate and, if adopted consistently, would 

 materially simplify the presentation of this part of crystal 

 optics. The importance of this fact was first realized by 



