212 SUMMARY OF CUREENT RESEARCHES RELATING TO 



served, even with the dolomite, but \Adtli a little practice the eye 

 becomes accustomed to these differences, and the two substances can be 

 readily distinguished by this method. 



« 



Index Ellipsoid (Optical Indicatrix) in Petrographic Microscope 

 Work.*- — In considering this matter, F. E. Wright emphasizes the 

 importance of presenting the subject of microscopical petrography 

 consistently from tlie view-point of the index ellipsoid (optical indi- 

 catrix), as applied to wave-front normals. The various optical proper- 

 ties employed in practical petrographic microscopic work can be best 

 described and explained systematically by means of the index ellipsoid. 

 The use of the so-called " axes of elasticity," a, b, z, or X, Y, Z, in this 

 connexion is confusing, and only adds to the difficulties encountered 

 by the observer in mastering the subject. They should accordingly be 

 abandoned, and the French usage of naming the principal axes of the 

 index ellipsoid (a, jS, y, or ?i^, 7i,,„ n,,) adopted. This applies in particular 

 to the diiferent modes now in vogue for expressing extinction angles. 

 For a given crystal face an extinction angle is simply the angle between 

 a definite crystallographic direction on the face and one of the axes, 

 a or y', of its optic ellipse, and this fact should be indicated in the 

 expression for the extinction angle. To introduce " axes of elasticity " 

 (it, z, or X', Z') in this connexion is not only needless but less direct, 

 as it introduces entirely new conceptions which experience has shown 

 only tend to bewilder the student. Clear concise modes of expression 

 and simple methods of attack are as essential in petrology as in other 

 sciences, whose development is often directly dependent on the care 

 and attention given by its workers to these features. 



Media of High Refraction for Refractive Index Determinations 

 with the Microscope ; also a set of Permanent Standard Media of 

 Lower Refraction, f — H. E. Merwin has made a number of experimental 

 studies for the purpose of extending the conditions under which deter- 

 minations of refractive index by means of the Microscope can be made. 

 Such determinations require immersion media of standard refractive 

 index. Various immersion liquids have been in use for the determina- 

 tion of refractive indices over the interval 1 • H8 to 1 • 80 ; mixtures of 

 amorphous sulphur and selenium have been found useful over the range 

 (for sodium light) 2*1 to 2 '4. The author's efforts have l)een devoted 

 to filling the gap I'HO to 2' 10, and to extending the series beyond 

 2*4 N.A., or, in special cases, particularly when a refractometer is not at 

 hand for standardizing the liquids, to replacing media hitherto used. 

 Although the refractive indices can be determined to three places of 

 decimals, it is seldom that in determinative mineralogical work results 

 closer than O-Ol are of practical use, owing to the complex character of 

 most minerals. For full information the author's directions and statis- 

 tical ta])les must be consulted, but the following outline of results may 

 be of service. 



* Amer. Journ. Sci. xxxv. (1U13) pp. 133-8. 



t Jouni. Washington Acad. Sci., iii. (1918) pp. 35-40, 



