CHARACTER OF THE PRINCIPAL ZONE. 73 
by the quartz plate (Fig. 45). On both sides of the line of exact compensa- 
tion the interference colors rise gradually without abrupt change at any 
point. This device is equivalent to a wedge which has been ground down 
to an infinitely thin edge; at the line of exact compensation it has no notice- 
able effect on the light- waves passing through, with the result that the 
interference color of a mineral seen through the wedge at this point appears 
the same as though no wedge intervened. If the wedge be slowly withdrawn 
from this position, the interference color of the mineral is observed to rise 
or fall gradually without an abrupt change at the start. 
Near the center of the wedge is a point for which the path difference 
between the two emergent waves is \ X. This part of the wedge can there- 
fore be used in place of the ^ undulation mica plate. 
To render this combination wedge as useful as possible, it is fitted in a 
metal frame (Fig. 46) of the same outer dimensions as the ordinary wedge 
and with it in the same frame a short quartz (or selenite) sensitive-tint plate 
is placed at one end. A space b is left free and is thrown into the field when 
the wedge is not in use. To steady the motion of the wedge and also to 
mark the position of the open space b a small steel spring with small rounded 
tip is fitted to the draw tube of the microscope and presses against the 
metal frame of the wedge. This arrangement combines, therefore, the 
three attributes ordinarily used, obviates one of their defects, and is attached 
permanently to the microscope. 
As noted in the Introduction, the practice of inserting wedges and plates 
above the objective does not improve the optical system, but tends rather 
to decrease materially the definition in the image (especially if high powers 
be used) as well as to disturb the focus; these annoying features can be 
remedied by inserting the sensitive plate or wedge between the polarizer 
and condenser lens. (Plate i, Fig. 3.) The sensitive plate mounted in a 
metal frame fits in a collar which can be rotated about the axis of the micro- 
scope, thus permitting the plate to be turned rapidly from one quadrant 
to the second and allowing the rise and fall of the interference colors in a 
crystal section or an interference figure to be observed in quick succession 
without disturbing the section itself, as is ordinarily done by rotating the 
stage.* If a wedge be inserted in place of the sensitive tint plate, the sub- 
stage diafram must be stopped down as far as possible or a slit aperture 
introduced in place of the diafram in order that the interference effects 
due to a single thickness of the wedge be obtained. 
As the relative value of the ellipsoidal axes in the wedge or plates is 
definitely known, the relative value of the ellipsoidal axes in a given crystal 
section is obtained by direct comparison with the standard plate or wedge, 
thus determining the optical character of the principal zone or the relative 
value of the ellipsoidal axis parallel to the direction of elongation of the 
crystal. 
*In case the mineral section is exceedingly minute, and the interference color, dark gray of the first order 
and hardly perceptible, the brilliantly illuminated field, produced on insertion of the wedge in the diagonal 
position, may be sufficient to veil the delicate color phenomena which appear on the mineral grain. In 
such instances the wedge below the condenser should be rotated until its ellipsoidal axes almost coincide 
with the principal nicol planes. The illumination resulting from the wedge is then hardly perceptible, 
while the change in interference tint from red to blue is clearly marked in the minute mineral plate or lath 
in the diagonal position. 
