86 



It is otherwise with the interjacent parties. A ray, for instance, 

 impinging on ilf, falls obliquely down on the reflecting plane, 

 and the direction of oscillation PP forms an angle of 45° with 

 the plane of incidence. By the reflection the ray is divided 

 into two components, one of which oscillates in the plane of 

 incidence, the other at a right angle to this plane; the greater 

 part of the former is absorbed, while the latter is almost entirely 

 reflected. After the reflection the two components, which get, 

 moreover, a greater or smaller difference of phase dependent 

 on the angle of incidence, are joined into an elliptically polarized 

 ray, which closely approaches to being rectilinearly polarized, 

 if the ray is reflected at the principal angle of incidence. In 

 the latter case the direction of oscillation of the reflected ray 

 is J- OM, and will thus, without any considerable weakening, 

 pass the analyzer. Otherwise the major axis of the ellipse will 

 in all instances be most nearly J- on this direction in the 

 whole upper half in the field of view, while in the lower part 

 it is 7^ this direction. 



If a face of ilvaite, the best one will be è{010}, is placed 

 under the objective in the position shown in the figure, the 

 upper part of the field of view will be a strong yellow and the 

 lower part green; if the table is turned 45°, the colours dis- 

 appear entirely; by a further turning of 45° they are seen 

 again, but have changed their places. 



A considerable number of other minerals that are highly 

 pleochroitic in reflected light, as for instance tourmaline, epidote, 

 different pyroxenes and amphiboles, have been placed in the 

 same way under the objective, but in none of these cases has 

 it been possible to discover any colour. Surface-pleochroism 

 must consequently be of very rare occurrence among the 

 minerals; outside the minerals it is, as is well known, found 

 very finely developed, for instance in magnesiumplatinocyanide, 

 and allied salts. 



Pleochroism in transmitted light. Preparations were made 



