GEOPHYSICAL LABORATORY. 147 



Microscopic examination of the mix-crystals showed that they were homo- 

 geneous or had indication of zonal structure. 



The crystallographic examination proves that the two phosphates are 

 isomorphous. The thermal examination further shows that they are perfectly 

 miscible in the solid state and that the system Mg2P207-Mn2P207 belongs to 

 Roozeboom's Type I. 



(18) The determination of the relative refringence of mineral grains under the petrographic 



microscope. Fred. Eugene Wright. J, Wash. Acad. Sci., 4, 389-392 (1914). 



The standard methods for the determination of the relative refringence of 

 mineral grains under the microscope are convenient and entirely adequate 

 for ordinary purposes; but where great accuracy is required they are less 

 satisfactory and are difficult to apply because of the eye-strain involved. A 

 new method of two-fold oblique illumination is proposed which enables the 

 observer to reduce the field illumination at will and thus to obviate the eye- 

 strain. In this method two straight-edged metal stops are used, the first below 

 the condenser and the second in the conjugate image plane of the first above 

 the condenser. By adjustment of these stops it is possible to decrease the 

 intensity of illumination to any extent and thus finally to exclude all light 

 except that refracted by the individual mineral grains; conditions are then 

 attained which are analogous to those first used by Topler for the detection of 

 optical inhomogeneity in glass. The sources of monochromatic light best 

 adapted for refractive-index work with the petrographic microscope are 

 described briefly. 



(19) The optical character of the faiut interference figure observed in high power objectives 



between crossed nicols. Fred. Eugene Wright. J. Wash. Acad. Sci., 4, 301- 

 309 (1914). 



The appearance of a faint, apparently uniaxial interference figure in a high- 

 power microscope objective between crossed nicols is a matter of common 

 observation. The first correct explanation of the phenomenon was given by 

 Rinne in 1900, but no explanation of the apparently optically positive character 

 of this interference figure has heretofore been given. In the present paper the 

 explanation of this conversion of an isotropic substance, like glass, into an 

 apparently uniaxial optically positive substance is presented in some detail 

 and rendered evident both by a series of experiments and by theoretical com- 

 putation. The plane of vibration of the incident plane-polarized light waves 

 is rotated at the steeply inclined surfaces of the lenses in the objective. The 

 amount of the rotation at a particular point is dependent on the inclination 

 and azimuth of the surface of the glass with respect to the plane of the analyzer, 

 the refractive index of the glass, and the wave-length of light employed. The 

 combination of these factors gives rise to the phenomena observed. 



(20) A new half-shade apparatus with variable sensibility. Fred. Eugene Wright. J. 



Wash. Acad. Sci., 4, 309-313 (1914). 



The apparatus consists of two strips of plane parallel glass cemented to the 

 two sides of a 45° total reflecting prism in such a manner that the ends of the 

 strips extend 6 or 8 mm. beyond one end of the prism. The hypothenuse face 

 of the prism is mounted on an axis supported in a brass cylinder in such a way 

 that the common edge of the glass plates is normal to the axis of rotation. If 

 the glass plates be observed between crossed nicols, the plane of vibration of 

 the light-waves transmitted through them can be rotated through a small 

 angle by turning the device about the axis, the angle of rotation of the plane of 

 vibration of the light-waves in the one plate being equal but opposite to that 

 in the second glass plate. This device furnishes a simple and inexpensive 

 method for constructing an effective half-shade apparatus with variable 

 sensibihty. 



