192 



MINERALOGY 



mineral section. When in position this lens yields a cone of light, 

 all rays of which will not be transmitted parallel to the optic axis 

 on passing a section, cut parallel to the base, but only those in the 

 axis of the cone of light, Fig. 339, oc. This ray will be perpendicular 

 to the section, or parallel to the optic axis, and will be transmitted 

 by the section without double refraction ; its vibration plane is 

 that of the polarizer PP', and it is therefore extinguished by the 

 analyzer, and the spot c representing the optic axis will be dark. 

 All other rays entering the section, the paths of which are not 

 parallel to oc, are doubly refracted. One ray is retarded behind 

 the other, the amount of retardation depending upon the double 

 refraction of the section and the length of the path that the rays 

 travel through the section. As the paths differ and increase in 

 length with the change of the inclination of the rays to the section 

 SS', the path followed as bbi is longer than cci. The differences 

 of phase of the two rays will increase from the center c, where it is 

 zero, equally in all directions on the surface of the mineral section. 

 There will be a point X where this difference of phase will be one 

 wave length ; the cross section of all such waves at right angles 

 to oc will form a circle, as indicated on the diagram. These two 

 waves emerging at X when viewed with the analyzer will be brought 

 to vibrate in one plane and in a position to interfere ; as the ana- 

 lyzer adds one half wave length to the difference, the effect will 

 be at the point X and on the circle with a radius X c, a difference of 



phase of 3/2 X, or of darkness if 

 monochromatic light is used. 



Somewhat further away from 

 c than X there will be a circle 

 where the difference . in phase 

 will be 2 X as appearing in the 

 analyzer, and a concentric re- 

 gion of maximum light will 

 mark the region, followed by 

 a concentric ring of darkness, 

 etc. The concentric rings of 

 light and darkness will be nar- 

 row or close together as the 



angle of the inclination to the ray grows smaller or the ray is more 

 inclined. In viewing the interference figure in the microscope, two 

 dark areas, parallel to the vibration planes of the nicols, will be 

 noted as crossing the center or optic axis at right angles and divid- 



FIG. 340. 



