WED GEVFOR THEDVPETROGRAPHICAL MIECROSGOPRE, 35 
above the objective. The small rounded tip at the end of the 
spring presses up against the metallic rim of the wedge, and at 
the point where the space is in the center of the field snaps into 
a small pit made in the rim for the purpose. 
The weak point of the ordinary wedges, that the interference 
colors rise or fall abruptly on their insertion, is thus remedied, 
and the three plates, quarter-undulation plate, red first order, 
and wedge are united to one wedge which remains in its place 
on the microscope, and which it is not necessary to remove 
after each determination.' 
FRED EUGENE WRIGHT. 
MICHIGAN COLLEGE OF MINES, 
Houghton, November, 1901. 
*The above wedge was first described in a footnote, p. 275, of ‘‘ Die foyaitisch- 
theralitischen Eruptivgesteine der Insel Cabo Frio, Rio de Janeiro, Brasilien,’ von 
Fred Eugene Wright,” TSCHERMAK’s AZinerolog. petrogr. Mittheilungen. Bd. XX, 
pp. 233-306. The combination wedge is made by Voigt & Hochgesang, Gottingen, 
Germany. Price, 21 M. (ca. $5 if quartz be used. [The wedge can also be obtained 
from Bausch & Lomb.] The price is still less if selenite be substituted for quartz). 
In ordering, the dimensions of the aperture into which the wedge is to be inserted 
should be given, also whether the wedge is inserted into the microscope tube parallel 
to the horizontal cross hair, or at an angle of 45° with same. 
