•Jos /•'. /•;. Wright — 7 'runs mi \s'.s'io)i of Light through 



A device to aid in the graphical solution of optical problems 

 involving the use of the stereographic projection. 



In the measurement of optic axial angles in convergent polar- 

 ized light,' and also in all measurements by means of the uni- 

 versal stage methods, the stereographic projection plat of 

 Prof. Wulff has proved a useful and necessary adjunct, the 

 angular values of observation being plotted directly on thin 

 transparent paper placed above the plat and held in the center 

 by means of a needle. This needle, however, is not entirely 

 satisfactory, since it does not hold its place rigidly and tends 

 thereby to injure the stereographic plat below. To overcome 

 this difficulty the writer has constructed the device of fig 18 



Fig. 18. 



C D 



E 



(one-eighth actual size). A heavy brass bar fits into two end 

 blocks of brass, A and B ; at its center a small hollow brass rod, 

 C, containing a needle backed by a spring is introduced. By 

 this device the needle is rigidly supported in a vertical posi- 

 tion, and as the distance between the end blocks A and B is 

 44 cm , there is more then sufficient space available for the pro- 

 jection plat and the overlying drawing. The writer has used 

 this device for several years and has found it satisfactory and 

 a time saver. 2 



Summary. 



Minerals are determined under the microscope by the effects 

 they produce on transmitted light waves. Plane polarized 

 light waves are ordinarily used and examinations are made 



1 For determining the Mallard constant of the microsope, which is required 

 in the measurement of optic axial angles by means of the microscope, Dr. J. 

 S. Flett of London uses a Zeiss Abbe apertometer. His method is simple 

 and accurate and is sixperior to any method yet suggested. He introduces 

 the micrometer scale in the ocular as usual and then determines the divi- 

 sions covered by the different angles of the apertometer. Since with this 

 device any angle can be instantly set off, an objective can be calibrated 

 rapidly for all possible angles within the field of vision, and an empirical, 

 correct table prepared which is independent of the Mallard formula, thas 

 obviating all errors due to a lack of correction in the objective lenses. 



- Recently, Prof. Nikitin has had constructed a graduated porcelain hemi- 

 sphere (made by E. Fuess, Steglitz, Berlin, Germany) which the writer has 

 found very satisfactory in optic axial angle projections and slightly more 

 accurate than the projection plats, chiefly because of its lack of distortion 

 toward the margin and consequent acute angled intersections of great circles. 

 This hemisphere has the advantage of serving as a model in the study of 

 optical phenomena and is a useful piece of apparatus for the petrological 

 laboratory. 



