ge:ophysical laboratory. 103 



The optical and crystallographic characters which thus serve in the micro- 

 scopical examination of minerals can be grouped into two classes, those of 

 the first class (crystal habit, color, pleochroism and absorption, optical char- 

 acter of elongation, optical character of the mineral, and dispersion of the 

 optic axes and bisectrices) being ascertained usually by direct observation 

 without measurement, while for the second class (cleavage angles, extinction 

 angles, optic axial angle, refractive indices, and birefringence) numerical 

 values obtained by actual measurement are required. The properties included 

 in the first group are used as a general rule only in an essentially qualitative 

 way in microscopic work and can usually be recognized at a glance. For 

 these the ordinary petrographic microscope suffices. But in the second group 

 the quantitative element predominates ; and, as such, demands accurate, pre- 

 cise measurements. In ordinary petrographic work, however, these proper- 

 ties are only very roughly measured, and are then expressed in general terms, 

 as optic axial angle, large ; birefringence, strong, etc., actual numerical data 

 being rarely given. But with the increased knowledge of rocks and minerals 

 thus attained, the demand for data which are precise and quantitative in 

 character, rather than qualitative, has become more imperative, with the re- 

 sult that at the present time a thorough petrographic investigation should 

 contain accurately determined optical constants of each of the rock-forming 

 minerals examined, and, in critical instances, the probable error of each 

 determination. 



The present publication aims to ofi:'er a connected presentation of the entire 

 investigation of the petrographic microscope so far as it has been carried, in 

 which the different methods are coordinated and the significance and useful- 

 ness of each particular method is made to appear in its proper relation. An 

 effort has been made to establish the limits of accuracy of each method as 

 ordinarily used, and also the limits of accuracy theoretically attainable in 

 measurements of this kind. 



It may be stated, as a result of experience, that on clear individual grains 

 measuring from o.oi to 0.03 mm. in diameter, all the optic properties ordi- 

 narily employed in the petrographic microscopic investigation of minerals 

 in the thin section can now be determined with a satisfactory degree of 

 accuracy. 



The introductory chapter contains a statement of the general theory of 

 the microscope and the uses of its individual parts and accessories. Chapter I 

 includes the application of the microscope in the determination of proper- 

 ties of the first class (color, pleochroism, absorption, crystal habit, optical, 

 character, dispersion, etc.), which do not admit of numerical measurement. 

 The properties of the second class which admit of accurate determination 

 and numerical expression are treated in the following chapters : Chapter II. 

 Refractive indices ; III, Birefringence ; IV, Extinction angles ; V, Optic axial 

 angles. 



(22) Melting temperatures of sodium and lithium metasilicates. F. M. Jaeger. Jour. 

 Wash. Acad. Sci., i, 49. 191 1. 



Several carefully made and analyzed preparations of sodium and of lithium 

 metasilicate were studied in this laboratory with a view to establishing a con- 

 venient standard melting temperature in the interval between the melting- 

 point of copper (1082.6°) and that of diopside (1391°). 



The melting-point of lithium metasilicate was found to be 1201° ±1°, the 

 uncertainty depending upon the care with which this synthetic preparation is 

 made up. Sodium luetasilicate is more viscous and undercools in solidifying 

 to such an extent that no fixed solidifying temperature can be determined by 



