STUDY OF ORES AND METALS. 403 



tropism ; that for this determination methods based either on the 

 contrast in intensity of the two reflected components or on the 

 amount of rotation of the plane of polarization on reflection of the 

 incident plane-polarized light may be employed ; that methods based 

 on the phase difference between the two reflected components are 

 in general of little value because of the small differences in phase 

 which ordinarily result for a relatively large change in birefringence 

 or biabsorption. 



Several new methods are described for detecting anisotropism in 

 opaque substances ; of these, that requiring the use of the bi-quartz- 

 wedge-plate is the simplest and has been found in practice to be 

 superior to any heretofore suggested. 



Theoretical. 



Light waves on passing through homogeneous material are ab- 

 sorbed to a greater or less degree. In transparent media the amount 

 of absorption (in the visible spectrum) is relatively slight and can 

 be neglected for most purposes ; but in absorbing media there is an 

 appreciable weakening in intensity even in thin plates, which gives 

 rise to the phenomena of absorption. No body is, however, per- 

 fectly opaque (perfect absorber) or perfectly transparent, and the 

 terms transparent and absorbing are relative terms which express, 

 in a general way, the degree of absorption in the visible spectrum. 

 The conditions may, of course, be reversed in the infra-red or ultra- 

 violet. The experimental law of absorption, as expressed by Lam- 

 bert, states that in a homogeneous medium each layer of equal thick- 

 ness absorbs an equal fraction of the light transmitted ; if the layers 

 be considered to be one molecule deep, as in some crystals, then each 

 layer absorbs the same percentage of the light which passes through 

 it. In other words, for a layer of given thickness the intensity of 

 light transmitted (total light less absorbed light) is proportional to 

 the intensity of the incident light or 



f = /. I = /..-'. (I) 



where /q^ intensity of incident light, I, intensity of transmitted 

 light, /, thickness of layer, and m, the absorption modulus {i.e., the 



