82 F. E. Wright — Oblique Illumination, etc. 



interference figures from the same plate in convergent polar- 

 ized light. The study of mineral plates bj the method of ob- 

 lique illumination is of value because it impresses the mind of 

 the observer with the interdependence of optical and crystallo- 

 graphic properties. It is, however, highly important that the 

 observer realize the essential agreement between the phenom- 

 ena observed in oblique illumination and those seen on interfer- 

 ence figures in convergent polarized light. In the interference 

 figures the interference color phenomena are seen at a glance, 

 and if they be studied with reference to the position of the 

 mineral from which they are obtained, all of the conclusions 

 to be ascertained by means of the method of oblique illumin- 

 ation can also be derived with even greater facility from the 

 interference figure. For the study of interference phenomena 

 the method of oblique illumination does not offer any special 

 advantages over the convergent polarized light method, but it 

 does present certain disadvantages in manipulation and in the 

 distinctness of the phenomena observed which cannot be disre- 

 garded entirely. This is especially true if oblique illumination 

 be obtained by use of a stop in the eye circle of the ocular as 

 recently suggested by Schneiderhohn. The best method for 

 obtaining oblique illumination is that outlined under method 2 

 above. 



Measurement of Extinction Angles. 



In the foregoing pages the phenomena produced by oblique 

 illumination have been discussed in some detail. Attention 

 should also be directed to an obvious but important fact, too 

 often disregarded in petrographic microscope work, that for the 

 accurate measurement of extinction angle central illumination 

 by parallel plane-polarized light is highly essential. Satisfac- 

 tory measurements of extinction angles cannot be made when 

 the section is illuminated by a strongly convergent cone of 

 light, incident under all angles and in all azimuths. The 

 reason for this is apparent from the discussion above and 

 requires no further comment. 



Geophysical Laboratory, 



Washington, D. C, 



October, 1912. 



