EXTINCTION ANGLES. 
135 
of this wedge (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 and of the Koenigs- 
berger plate) have been found specially useful with minerals showing inter- 
ference 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 Plate 
6, Fig- 3, and is viewed by the Ramsden ocular. 
CALDERON* CALCITE PLATS. 
This device also is placed in the focal plane of the ocular and consists of 
two adjoining calcite plates so cut that the direction of extinction in each 
plate makes an angle of about 3.5 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 an intervening crystal plate, 
lights the entire field under crossed 
nicols with a dull gray tone. If a crys- 
tal plate, whose lines of extinction do 
not coincide with the principal nicol 
planes, is then observed, the field ap- 
pears 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 F IG . 80. 
claims an accuracy of 
=*= 2' with this ocular, but for a single determination and for general prep- 
arations the probable error is considerably larger (10' to 15')- The 
principle on which this method is based is evident from the intensity for- 
mula, 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 3. 5 +9 and for 
the second 3.5 0) and this produces at once a marked difference in the 
intensity of illumination. 
QUARTER-UNDULATION PLATE OF H. TRAUBB.f 
This plate consists of two adjacent quarter-undulation mica plates so cut 
that the optic axial plane of each includes an angle of 3.5 (Fig. 81) with the 
common line of junction, and for slight deviations of a crystal plate from 
its correct extinction direction the two halves appear unequally lighted, and 
only when the crystal is precisely in its position of total extinction do the 
halves show the same intensity of illumination. 
*Zeitschr. Kryst.. 2, 69-73. 1878. The calcite twin plates of a Calderon ocular were tested by the writer 
and found to be inaccurately ground. The plate WAS 3.18 mm. 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 ,\.i on the 
other. Extinction angles measured with this ocular, using the junction line of the plate as the line of ref- 
erence, 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. 
tNeues Jahrbuch, 1898, I, 251. 
