330 Shepherd, Ranhin, Wright — Binary Systems of 



natural minerals with their varying admixtures, from the chemi- 

 cally pure ultimate types. By the microscopic examination of 

 preparations of different composition in a given series, the 

 number of compounds in the series can be ascertained, the dif- 

 ferent phases in which any given compound appears, and also 

 the extent to which any particular compound takes up an 

 adjacent compound in solid solution. The exact determination 

 of the optic constants of the different members of the series 

 furnishes, morever, data which permit any one of them to be 

 recognized, even in the presence of others. From a textural 

 standpoint, the formation of eutectics should give rise to 

 special textures, and in some instances it has been observed to 

 do so, but as a rule crystallization in silicates at high tempera- 

 tures does not proceed with sufficient regularity to produce 

 clearly defined textures, and in most instances differences 

 between the crystals first to form (phenocrysts) and the por- 

 tions last to crystallize out (groundmass, eutectic), are not well 

 marked and the attention of the observer is directed chiefly to 

 the crystal development of the individual' crystals themselves. 

 This condition, together with the fact that the preparations are 

 usually examined in the powder form, tends greatly to reduce 

 the value and usefulness of textural evidence in the micro- 

 scopic investigation of such preparations. 



The optic properties which are made use of in the micro- 

 scopic examination of artifical products are the usual ones 

 employed in mineral determinations in rock sections, and 

 would require no comment at this point were it not for the 

 fact that artificial preparations are usually much finer-grained 

 than rock sections, and that in addition it is necessary to know 

 the degree of accuracy of all measurements on such products. 

 This has led the writer to make practical tests of available 

 microscopic methods to establish their accuracy and adaptabil- 

 ity to the new conditions found in artificial melts. In the 

 course of the general investigation, several new methods were 

 devised with special reference to the new requirements and 

 have proved satisfactory. At the present time, the following 

 methods and optic properties have been found most serviceable 

 in the study of artificial preparations. 



(1) Refractive indices. — In powder preparations the refrac- 

 tive indices are most readily determined by the immersion 

 method (Schroeder van der Kolk)* in refractive liquids of 

 known refractive index. On favorable clear grains the refrac- 

 tive indices can be determined by this method on grains 

 measuring even less than -01 mm in diameter and with a prob- 



* J. L. C. Schroeder van der Kolk, Zeitschr. f. wiss. Mikrosk., viii, 458, 

 1898; also F. E. Wright, T. M. P. M., xx, 239, 1900; this Journal (4), 

 xxii, 385, 1904 ; xxvii, 35, 1909. 



