87*2 F. F. Wright — Measurement of Extinction Angles. 



(10-15'). The principle on which this method is based is evi- 

 dent from the intensity formula, for in case the ellipsoidal axes 

 of the plate do not coincide precisely with the principal nicol 

 planes, they make unequal angles with the optic ellipsoidal axes 

 of the calcite (in the one half, this angle is 3J° + and for the 

 second 3-J° — 6) and this produces at once a marked difference 

 in intensity of illumination. 



Quarter-undulation plate of II. Traube.j- — This plate con- 

 sists of two adjacent quarter-undulation mica, plates so cut that 



Fig. 9. 



the optic axial plane of each includes an angle 3-|° with the 

 common line of junction and for slight deviations of a crystal 

 plate from its true position of extinction, the two halves appear 

 unequally lighted, and only when the crystal is precisely in its 

 position of zero extinction do the halves show the same intensity 

 of illumination. 



Twinned Selenite plate. — The use of a twinned selenite 

 plate has been recommended recently by E. Sommerfeldtf for 

 the accurate adjustment of the ocular crosshairs to the planes 

 of the nicols. But the same twins can be made to serve admir- 

 ably in the measurement of extinction angles. The extinction 

 angle wdiich the ellipsoidal axis makes in each plate with the 

 twinning plane is 37-J- , and if the twinning line on such a plate 

 be turned to the diagonal position with the crossed nicols, the 

 extinction angle on each side of the nicol measures 45°— 37-J 

 = 7-J-°, but in the opposite halves different optic ellipsoidal axes 

 are adjacent the principal nicol plane. The net result of this 

 arrangement is a change in intensity dependent not only on 



* Neues Jahrbuch, 1898, i, 251. 



fZeitschr. f. wissensch. Mikroskopie, xxiv, 24-25. 1907. 



