STUDY OF ORES AND METALS. 445 



crystal, therefore, of such weak birefringence or weak biabsorption 

 such that the difference in intensity between the reflected components 

 is less than 1.5 per cent, appears isotropic. In transmitted light dif- 

 ferences in birefringence of only 0.02 per cent, of the refractive 

 index are readily detectible ; therefore the methods based on dif- 

 ferences in intensity of the reflected components of vertically inci- 

 dent light are 50 or more times less sensitive in the detection of 

 anisotropism than the methods based on the phenomena of plane- 

 polarized, transmitted light waves. 



In case the vertically incident light be plane-polarized, the dif- 

 ference in amplitude of the reflected components normal or parallel 

 to the plane of symmetry causes a rotation of the plane of polariza- 

 tion and this can be detected and measured by any one of a number 

 of devices in common use by petrologists. Of these Koenigsberger 

 adopted a Biot sensitive tint plate and used it in a qualitative way to 

 detect anistropism. Hanemann improved Koenigsberger's method 

 by adopting the Biot-Soleil sensitive tint bi-plate and obtained 

 thereby a color contrast in two adjacent fields. The ordinary Bert- 

 rand eyepiece which is in common use by petrologists is suggested 

 above as a still further and equally simple method for detecting 

 anisotropism. 



A still more sensitive and better method is, however, to use the 

 writer's bi-quartz-wedge-plate in which the sensibility is variable and 

 can be adjusted to meet the conditions of illumination and thus to 

 produce the most favorable conditions for extreme sensitiveness and 

 consequent accuracy of results of measurement. This method is 

 simple and is the most sensitive at present available for detecting and 

 measuring anisotropism in opaque substances. The degree of aniso- 

 tropism is indicated by the amount of angular rotation on reflection 

 of the plane of polarization of the vertically incident light waves. 

 The accuracy of these methods depends on the threshold limit of 

 vision (least intensity of light which the eye can detect) ; this is dif- 

 ferent from the least perceptible difference in intensity of illumina- 

 tion of two adjacent fields on which the sensitiveness of the first 

 group of methods depends. 



The sensitiveness of the second group of methods can be in- 

 creased by increasing the intensity of the light source and also by 



