OPTICAL PROPERTIES OF CRYSTALS 



negative ( ). The arrows in the diagram are reversed. Care 

 should always be taken that the same relative positions of the wedge 

 ;ind the:ixi:il plane exist ; for if the slow ray of the wedge i- intro- 

 dueed at right, angles to the plane of the optic axes, all motions of 

 the color bands are reversed and the sign may be taken opposite to 

 what it really is. Also in sections of negative crystals after the 

 point of compensation has been reached and the wedge is still ad- 

 vanced, t he effect is as if thickening t he section, or a positive crystal. 



Measurement of the angle between the optic axes. I. The ap- 

 proximate value of 2 E may be obtained by measuring the distance 

 between the two poles of the hyperbolas in the interference figure 

 of a section perpendicular to the acute bisectrix and in the 45 

 position with a micrometer eyepiece. Placing this value at 2 d, 

 then sin E = d/C, where C is a constant for the combination of 

 lenses used, and may be determined by a section in which the angle 

 between the opfck axes is known, as aragonite. 



II. After the three indices of refraction have been determined, 

 the angle 2 V rnjuy be calculated from the formula, 



cos V = 



As the value of 2 V is influenced considerably by variations in the 

 fourth decimal place of the value of the indices of refraction, this 

 method is not as accurate as the direct determinations of the angle. 



III. In the direct determination a section of the crystal is 

 required, cut perpendicular to the acute bisectrix. The section is 

 mounted in the axial angle goniometer, with the plane of the optic 

 axis at 45 6 to the vibration planes of the nicols. One hyperbola is 

 brought tangent to the hair of the eyepiece and a reading taken ; 

 then the second hyperbola is brought tangent to the hair and a sec- 

 ond reading taken ; the difference between the two readings is the 

 value of 2 E, the axial angle measured in air ; 2 V, the true axial angle, 

 may be calculated when the median index of refraction {3 is known, 



sin sinE 



sin r 



sin V 



sinE 

 p 



2 E is always greater than 2 V ; and when 2 V is large, the ray along 

 the optic axes is often totally reflected at the surface of the section, 



