A NEW COMBINATION WEDGE FOR USE WITH THE 
PEDROGRAPEIEAE MICR@OSCORE: 
TurREE different methods are at present in general use for 
determining the optical character of a mineral with the petro- 
graphical microscope. The first of these employs a quarter-mica 
undulation mica-plate, the second a selenite plate showing the 
interference color, red of the first order, and the third a quartz 
wedge whose interference colors run from gray of the first order 
to red of the third (Newton’s color scale). 
The underlying principle of these three methods is the 
same. For the examination of the optical interference figures 
in convergent light they answer their purpose well, but for the 
determination of the optical character of minerals in parallel 
polarized light they all exhibit one common failing. On insert- 
ing any one of them into the tube of the microscope the inter- 
ference color of the mineral in the slide rises or falls abruptly to 
some other interference color of the color scale. This jump 
Otrthe -interierence color, due) toy the. sudden change vor the 
distance between the two rays passing through the crystal, and 
caused by the insertion of the plate or thin edge of the quartz 
wedge, which in itself is so thick that it alone shows gray of 
the first order, is often sufficient to render the determination 
uncertain. In deeply colored minerals (in certain amphiboles 
and pyroxenes) this is particularly noticeable, for there the 
natural color of the mineral hides the interference color to a 
great extent. On inserting the plate or wedge, one observes a 
change of color in the mineral, but is often unable to distinguish 
whether the color has risen or fallen. 
This fault is easily remedied by combining a quartz (or 
selenite) wedge which shows gray first order to red third order, 
and in which the ray vibrating parallel to the long direction of 
the wedge has the greater velocity (a) with a selenite plate 
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