STUDY OF ORES AND METALS. 



421 



methods which involve direct comparison of two adjacent illumi- 

 nated fields (half shade principle). The accuracy of a single setting 

 obtainable with such methods is dependent on the sensitiveness of 



Fig. 3. In this figure is indicated the angular rotation in degrees (ordi- 

 nates) of the plane of polarization of vertically incident, plane-polarized light 

 of azimuth 6 = 45° on reflection from a crystal plate of least refractive index 

 111 (abscissae) and of birefringence n. — Wi (curves). 



the eye, or on the least perceptible difference in light intensity under 

 the conditions of observation. An extended series of measurements 

 by Koenig and Brodhun^ on the least perceptible increment (dif- 



^ Sitzungsherichte d. Acad. d. Wissensch. Berlin, July 26, li 

 melte Abhandl. und Physiolog. Optik " (A. Konig), 116-143. 



" Gesam- 



