350 F. E. Wright — Measurement of Extinction Angles. 



tion. This process has been carried to such refinement in certain 

 instances, as in the isomorphons series of plagioclase feldspars, 

 that it is now possible, from extinction angles alone, to deter- 

 mine very closely the actual chemical composition of the par- 

 ticular plagioclase in hand. 



For a given mineral plate in the thin section, the term 

 extinction angle signifies the angle between a known crystal- 

 lographic direction (cleavage line, or trace of a crystal face on 

 that plate) and one of its optic ellipsoidal axes or directions 

 along which it extinguishes when these directions are parallel 

 with the principal planes of the crossed nicols. In order to 

 ascertain this angle satisfactorily one must be able not only to 

 measure plane angles accurately, but also to locate correctly 

 the position of the optic ellipsoidal axes of the particular crystal 

 plate. The first condition is easily accomplished and demands 

 no special comment, while the second requirement is extremely 

 difficult to meet with any degree of satisfaction without great 

 expenditure of time. 



Many methods have been suggested by which the position 

 of the optic ellipsoidal axes of a given crystal section can be 

 located more or less exactly, and all are based on the fact that 

 when the optic ellipsoidal axes are parallel with the principal 

 planes of the crossed nicols the plane polarized light normally 

 incident from the lower nicol passes through the crystal plate 

 without changing its plane of vibration. In case the optic 

 ellipsoidal axes of the plate do not coincide with the planes of 

 the nicols, interference in general takes place and some light 

 passes through the upper nicol. The different methods pro- 

 posed have for their common object the rendering apparent 

 the extremely small percentage of light which thus emerges 

 from the analyzer when the angle of revolution of the crystal 

 plate from its position of absolute darkness is very small. 



Before considering in detail the different methods for accom- 

 plishing this result and their relative merits and defects, it will 

 be well to treat the subject mathematically and to derive the 

 formulas for the intensity of light with special reference to the 

 subject of extinction angles. This treatment is given in some 

 detail in the following paragraphs, since the deductions given 

 later are all drawn from these fundamental equations. 



Theoretical. 



Mathematical. — The phenomena of light are considered to 

 be produced by periodic changes or disturbances in the ether, 

 transverse to the direction of propagation. Different hypoth- 

 eses have been proposed which assign different properties to 

 this medium, but no one of the hypotheses yet suggested is 





