440 WRIGHT— POLARIZED LIGHT IN THE 



trast in adjacent fields and is accordingly more sensitive than the 

 Koenigsberger method. 



The Bertrand eyepiece which is in common use by petrologists 

 would serve the purpose as well as, and possibly better than, the 

 Soleil-Biot plate employed by Hanemann. 



Nezv Metliod. — The methods of Koenigsberger and of Hane- 

 mann are based on changes in color (sensitive tint) on rotation of 

 an anisotropic plate. In the case of strongly colored substances, 

 especially yellow minerals, the natural color dominates the field so 

 effectively that the sensitive tint is no longer present as such and 

 the slight changes in hue on rotation of the stage are only with dif- 

 ficulty detected and may be overlooked. The sensitiveness of these 

 methods varies therefore with the color of the substance under ex- 

 amination. It is also difficult to obtain a uniformly colored field 

 under average conditions of illumination, especially near the junc- 

 tion lines in the field. 



In measurements of this nature in which the effective conditions 

 vary within wide limits, whereas the sensitiveness of the eye (thres- 

 hold vision) is more or less fixed it is essential for the best results 

 that the sensitiveness of the device employed for the measurement 

 be variable so that the most sensitive conditions of observation can 

 be obtained. It is on this principle of variable sensibility that the 

 writer's bi-quartz-wedge-plate^^ was constructed. 



On substituting the bi-quartz-wedge-plate for the Biot plate of 

 Koenigsberger or the Biot-Soleil plate of Hanemann the most sensi- 

 tive arrangement for the detection of anisotropism in opaque sub- 

 stances is obtained. The method is exceedingly simple and requires 

 no apparatus in addition to that furnished with a research model 

 petrographic microscope. The observations are made either in white 

 or in monochromatic light. The degree of anisotropism is indicated 

 by the amount of angular rotation of the cap nicol required to pro- 

 duce equal intensity of illumination in the adjacent halves of the 

 bi-quartz-wedge-plate. 



Other devices, such as the half-shade Lippich prism, may be sub- 

 stituted for any one of the rotating quartz plates and wedges but 



^^ Am. Jour. Sci. (4), 26, 377-378, 1908; Carnegie Institution of Wash- 

 ington, Publication 138, 140-142, 191 1. 



