Using a detent spindle stage one can determine the optic axial angle, 2V, to a fraction of a degree and, simultaneously, the spindle settings that correctly orient the crystal’s indicatrix for measurement of refractive indices α, β, γ (or ε and ω ). Experimental procedure involves setting the spindle stage so that the reading Son its protractor scale is successively 0°, 10°, 20°, ⋯ 180°. For each such setting S, the microscope stage is rotated until the crystal becomes extinct. The microscope stage readings Ms for which crystal extinction is observed—namely M0, M10, M20M180— are analyzed by a least-squares computer technique which refines the coordinates for the two optic axes, calculates 22 V, and locates the three principal vibration axes with a potential accuracy not routinely available previously.

For an adularia from St. Gotthard, a set of 19 extinction measurements(M0, M10,… M180), here called trial (1), plus a second set of 18 extinction measurements (M5, M15,… M175), here called trial (2), observed at wavelengths 433 nm, 566 nm, and 666 nm yielded the following results:

Table
Λ(nm)Trial2VxXYZ
SESESE
433 (1) 63.66 87.57 86.56 172.96 143.19 0.13 126.60 
 (2) 63.58 87.70 86.74 173.3 143.09 0.14 126.72 
 Av. 63.62 87.64 86.65 173.15 143.14 0.13 126.66 
566 64.92 87.24 86.28 172.37 142.70 0.08 127.05 
 (2) 65.04 87.21 86.39 172.46 142.71 179.94 52.94 
 Av. 64.98 87.24 86.33 172.42 142.70 0.01 127.06 
666 65.18 86.69 86.11 171.49 143.14 179.58 53.42 
 (2) 65.74 87.52 86.30 172.54 143.27 0.26 126.48 
 Av. 65.46 87.10 86.20 172.20 143.20 179.92 53.47 
Λ(nm)Trial2VxXYZ
SESESE
433 (1) 63.66 87.57 86.56 172.96 143.19 0.13 126.60 
 (2) 63.58 87.70 86.74 173.3 143.09 0.14 126.72 
 Av. 63.62 87.64 86.65 173.15 143.14 0.13 126.66 
566 64.92 87.24 86.28 172.37 142.70 0.08 127.05 
 (2) 65.04 87.21 86.39 172.46 142.71 179.94 52.94 
 Av. 64.98 87.24 86.33 172.42 142.70 0.01 127.06 
666 65.18 86.69 86.11 171.49 143.14 179.58 53.42 
 (2) 65.74 87.52 86.30 172.54 143.27 0.26 126.48 
 Av. 65.46 87.10 86.20 172.20 143.20 179.92 53.47 

where we define S the spindle setting that would bring X, Y or Z into the plane of the microscope stage, and E as the angle between X, Y or Z and the spindle axis. Results for 2Vx, Y or Z clearly demonstrate dispersion of the optic axes (r > v). The relatively regular variation in the average, S and E angles observed from 433 to 666 mm for X the acute bisectrix, suggest it undergoes dispersion. The obtuse bisectrix,Z, appeare not to, but Ypossibly does. If so, the adularia must be optically monoclinic. However, the possibility remains that dispersion oF Z is so small as to be undetectable.

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