202 



MINERALOGY 



FIG. 351. Interference Figure of Ara- 

 gonite with the Plane of the Optic Axes 

 at 45 with the Planes of the Nicols. 



direction of the vibration planes of the two rays emerging at 



any particular point. In the plan or upper part of the diagram, 



let any point whatever, asR, be 

 taken, there will be two rays 

 emerge, vibrating in planes at 

 right angles. If from the point 

 R the lines RO and RO' be 

 drawn, they will be the traces 

 on the plane of the section of 

 the planes containing the ray 

 R and the optic axes O and 

 O'. The angle ORO' is bi- 

 sected by the trace of the 

 vibration planes on the plane 

 of the section, of one ray 

 emerging at R, that of the ex- 

 traordinary ray, as eei; the 

 vibration plane of the other 



ray will be at right angles to this plane, as ff . The two rays 



emerging at R, one vibrates parallel to ee', the other parallel 



to ff', both have components 



parallel to PP' and AA', the 



vibration planes of the nicols, 



and the point R in the field 



will be illuminated. When all 



points in the field are tested, it 



will be found that when the 



plane of the optic axis is paral- 

 lel to either vibration plane of 



the nicols, the dark area will 



be in the form of a cross, as 



represented in the photograph, 



Fig. 349. Let the stage of the 



microscope with the section be 



revolved 45 as in the dia- 

 gram, Fig. 350. O, O' are 



the optic axes and OO' is the 



trace of plane of the optic 



axes, now at 45 to the planes of the nicols. The dark areas in 



this position will be quite different from that illustrated in 



Fig. 349. If any point R be taken as before, and the vibration 



FIG. 352. Interference Figure of Ara- 

 gonite with the Plane of the Optic Axes 

 revolved slightly out of the Parallel 

 Position, showing the Formation of the 

 Hyperbola. 



