of Minerals in the Thin Section. 349 



of vibration (extinction angles) due to refraction from steeply 

 inclined plates and consequent elliptical polarization, are small 

 quantities of a low order of magnitude and can be disregarded 

 in general. 



The method of measuring the optic axial angle by means of opti- 



9.9. 23 







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ft" 



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Fig. 22. In the method illustrated by this figure, the visible optic axis 

 Ai is brought to coincide with the plane DCO and the extinction angle DOE 

 measured while the stage is in the horizontal position. A s must lie then in 

 the plane OB, the angle BOE having been made by construction equal to 

 DOE. The section is then revolved about OM (axis Vi) until the extinction 

 angle becomes 45°, in which case the plane OM contains A 2 , since Ai has 

 remained during this revolution in the plane DCO ; on turning the specimen 

 back to its original position, the line OM becomes the great circle CA 2 M and 

 the intersection of this great circle with the plane OB fixes A 2 definitely in 

 the projection. In practice, the great circle CA 2 M need not be drawn, since 

 on placing the tracing over the plat it is only necessary to find that small 

 circle A 2 A 2 ', the arc of which intercepted between OB and OM is equal to the 

 angle of revolution. 



Fig. 23. The general method of extinction curves shown in this figure is 

 applicable to all sections in which one optic axis Ai can be brought to coin- 

 cidence with the axis of the microscope. After the determination of the exact 

 position of Ai by means of optical curves the specimen is revolved about 

 H 3 until Ai coincides with the plane NO normal to the axis V] of the uni- 

 versal stage. The extinction angle MOE of the specimen in horizontal posi- 

 tion is then determined ; by construction EOA 2 is made equal to MOE ; the 

 specimen is then revolved about Vi a convenient angle (apparent angle 

 observed to be reduced to true angle^ and the new extinction angle MOE' 

 ascertained. In the new position, the optic axis is contained in the plane 

 OA 2 ', angle E'OA 2 ' having been made equal to MOE'. The exact position of 

 A 2 is then determined on the drawing on tracing paper by noting the small 

 circle of the underlying projection plat, whose arc A 2 A 2 ' intercepted between 

 OA 2 and OA 2 ' is equal to the angle of revolution. This determination can 

 be checked by drawing the great circle CF, which marks the position which 

 the plane OA 2 ' would assume were the specimen turned back to its original 

 position. In practice the position of A 2 is determined for different angles 

 of revolution about V x and the mean position of all determinations taken as 

 the most probable and correct location of A 2 . 



Am. Jour. Sci. — Fourth Series, Vol. XXIV, No. 142. — October, 1907. 

 24 



