F. E. Wright — Measurement of Extinction Angles. 371 



an ordinary combination wedge* showing the zero interference 

 band exactly in the center and green of the second order on 

 each end, cutting the same longitudinally in half parallel to 

 the ellipsoidal axes ; the edges were then polished and the 

 halves again reeemented, the one half, however, having been 

 rotated first through 180°, so that in the resultant combination 

 by wedge the phase difference of the adjacent half at any 

 point of insertion is always equal and opposite. By this method 

 the principle of the Bravais-Stober plate is extended to cover 

 interference colors from total darkness to blue green of the 

 second order, and to allow the observer to select an interference 

 color which, in combination with that of the mineral plate 

 examined, is most sensitive. The low gray tints of this w T edge 

 (particularly the dark band region on both sides of which the 

 interference colors rise and thus divide the field into four 

 Quadrants and produce an effect similar to that of the Bertrand 

 ocular) have been found specially useful with minerals showing 

 interference colors from red first order to blue second order. 

 This wedge is held in a brass carriage, which in turn slides 

 in the wedge holder shown in tig. 10, and is viewed by the 

 Ramsden ocular. 



Calderonj- Calcite plate. — This plate is also placed in the 

 focal plane of the ocular and consists of two calcite plates 

 placed side by side and so cut that the direction of extinction 

 in each plate makes an angle of about 3^° on opposite sides of 

 the common line of junction. The plate is so thick that the 

 interference color is white of the higher orders and when used 

 alone without intervening crystal plate, lights the entire field 

 under crossed nicols with a dull gray tone. If a crystal plate 

 whose lines of extinction do not coincide with the principal 

 nicol planes be then observed, the field appears divided into 

 two unequally illuminated halves and only when the extinction 

 directions coincide with the nicol planes is the intensity of 

 illumination in both halves equal. Calderon claims an accuracy 

 of +2' with this ocular, but for a single determination and for 

 general preparations the probable error is considerably larger 



* Compare F. E. Wright, Tschermak's Min. Petr. Mitth. xx. 238-306. 1901 ; 

 also Jour. Geol., 33-35, 1902. 



f Zeitschr. Kryst,, ii, 70, 1878.— The calcite twin plates of a Calderon ocular 

 from R. Fuess in Steglitz were tested by the writer and found to be inaccu- 

 rately ground. The plate was 3 "IS 111111 thick and cut at an angle of about 45° 

 with optic axis. The extinction angle in each half of the plate was measured 

 in convergent polarized light by means of the dark bar in the center of the 

 field and found to be + 4 - 4° on the one half and 3 -2° on the other. Extinc- 

 tion angles measured with this ocular, using the junction line of the plate as 

 the line of reference, would therefore be out 0'6° from this source alone. The 

 field of the ocular is. moreover, small and unfavorably lighted because of the 

 thickness of the plate and of the wide dark junction line across the center of 

 the field, which in turn disturbs the exact matching of the halves of the field. 



