54 The American Geologist. July, I89ri 
properties on a plane. Within these plates are expressed 
graphically the figures of maximum extinction and the direc- 
tions of their variations from pole to pole, also the curves of 
double refraction, their relations to the different axes of elas- 
ticity and the differences which the plagioclasesof the albite- 
anorthite series manifest. These tables are accompanied by 
a descriptive text and a full discussion of the methods. 
M, Fouque has recently added another chapter to the phys- 
ical examination of the feldspars. It is a publication of the 
Societe Fvanqaise de 3[ineraln(jie (Tome xvii, Nos. 7 and 8, 
1894) and differs from that of Levy principally in the choice 
of different planes within the crystal, from which to draw- 
optical properties. Whereas Levy employed the sections par- 
allel to the base and the brachypinacoid, Fouque has cut 
the crystal in planes perpendicular to its bisectrices. He thus 
avoids some of the difficulties inherent in the observation of 
the maximum extinction on cleavages in the base and brachy- 
pinacoid, although he encounters others which are, perhaps, 
at present equally formidable to the ordinar}^ student viz., the 
discovery of the plane of the optic axes and the cutting of 
the crystal perpendicular to the bisectrix. But the chief ad- 
vantage of the method of Fouque seems to lie in its avoidance 
of the actual observance of the maximum extinction of light. 
The eye is not sensible of small changes in the amount of 
light. There is need of making many readings and of taking, 
the average of these to warrant the observer in affirming the 
angle of greatest extinction. In place of a difference in light, 
Fouque substitutes a difference in form, which centers in the 
interference figure. When this figure is perfect it can easily 
be observed. When the hyperbolas are farthest removed from 
a cross in the field of the microscope the optic plane is at the 
angle of greatest extinction which can be read from the rim 
of the stage in the same manner as that obtained by maxi- 
mum extinction of light. 
There remain, of course, other directions in which crystals 
can be cut and examined, viz., those perpendicular to the op- 
tic axes, in which, as M. Fouque remarks, there are numerous 
properties of the various feldspars yet to be worked out. 
When these, and other microscopic physical properties by 
which the feldspars are presumably marked, are discovered 
