388 F. E. Wright — Measurement of Extinction Angles. 



crossed nicols until its halves show equal intensity of illumi- 

 nation, in which position the line of junction fixes the direc- 

 tion for one of the crosshairs of the ocular. 



By use of the bi-quartz wedge plate and the artificial quartz 

 twin plate or wedge, the adjustment of the nicols and also of 

 the crosshairs in the ocular is a matter of only a few moments, 

 and the method followed is theoretically and practically more 

 accurate than the other methods for adjustment which have 

 been suggested. 



3. A Device for Holding Small Crystals for the Purpose of 



Measuring Extinction Angles in Zones ; also for Measuring 



the Optic Axial Angle of such Fragments directly. 



In measuring the extinction angles of certain minute artificial 

 pyroxene . and other crystals in the prism zone, the writer has 

 had occasion to use the following holding device or finger, 

 which has proved both convenient and practical.* (Fig. 10 on 

 stage of microscope.) 



As indicated in the figure, it consists of two parts, a holding 

 or clamp device for the crystal itself, and a universal ball and 

 socket joint and centering plate for adjusting and centering 

 the crystal, and a vertical circle for reading any specified angle 

 of revolution of the crystal. A small crystal (l-2 mm in length) 

 thus held and adjusted is immersed in a drop of liquid of the 

 same refractive index and thus the disturbing phenomena of 

 refraction and total reflection eliminated. The liquid drop is 

 contained in the concave side of a planoconcave lens of 6 mm 

 diameter (ground for the purpose by the Scientific Shop of 

 Chicago), which in turn is held by an adjustable and support- 

 ing arm. 



In work with artificial preparations particularly it is often 

 desirable to measure extinction angles on certain minute 

 faces, or optical axial angles, of small fragments too minute for 

 the optic axial angle apparatus and not suitable for measure- 

 ment by one of the microscopic methods,f and this device has 

 been made to fill that want. After adjustment and immersion 

 in a liquid of refractive index fi the optic axial angle can be 

 read off directly on the vertical circle of this apparatus. 



Summary. 



The measurement of extinction angles of minerals in the 

 thin section is one of the most common methods of petro- 

 graphic microscopic practice, and at the same time one of 



* This apparatus was made in the workshop of the Geophysical Labora- 

 tory at the instigation of the writer, and can be constructed by any good 

 mechanic. 



f Compare F. E. Wright, Measurement of the Optic Axial Angle of Min- 

 erals in the Thin Section, this Journal (4), xxiv, 317-369, 1907. 



