of Minerals in the Thin Section. 



355 



of the section about V v Thus let a be the acute bisectrix (fig. 27) 

 and assume that one optic axis A, coincides precisely with the 

 normal to axis V, ; A 2 is then the second optic axis and angle 

 Aj/Ja equal to angle A n /3a, and A^a is the extinction angle. On 

 revolving, now, the section about V l7 the optic axial point A, is 

 brought to A\ and the extinction angle A'^E for the new posi- 

 tion of the section should bisect exactly the angle A'^A^. If 

 this be not the case and the extinction angle be too large or too 

 small, the section should be revolved about H 2 either counter 

 clock wise (A" 1 to A J or clock wise, A"\ to A v through a small 

 angle and a new set of measurements made, until after repeated 

 trials the corrected position is to be found for which observation 

 and construction agree precisely. The angle A^(3a is then half the 

 desired optic axial angle. 



27 



28 



In certain cases this method of placing the one optic axis A x in 

 the plane normal to the axis of revolution V, has been found 

 unsatisfactory, and a new method used which consists in first bring- 

 ing by trial the one optic axis to coincidence with the axis of revolu- 

 tion and then measuring the extinction angles for different angles 

 of inclination about V t (or V a ) and testing the results of observa- 

 tion and construction until they coincide. The method is shown 

 in fig. 28 and is so similar to the foregoing method (fig. 27) that 

 further description is unnecessary. 



For a section nearly normal to the obtuse bisectrix of a mineral, 

 both optic axes lie. again outside the field of vision and the optic 

 normal (3 cannot be brought to coincidence with the axis of the 

 microscope. The above methods do not apply, therefore, and new 

 ones are required to meet the new conditions and of these the 

 following has been found practicable by the writer. 



Place the universal stage in the primary position, the axis of 

 V 2 normal to that of V t and the circles H l5 H 2 , and H s , all in the 

 horizontal position ; determine the exact position of the obtuse 



