934 The American Naturalist. [November, 
age of iron, as shown by the subjoined mean of two closely agreeing 
analyses. 
SiO, 37.99 
A1,O, 29.53 
Fe,O, 5.67 
eO .58 
MnO 21 
CaO 23.85 
PEO 2.04 
99.82 
This corresponds with the accepted epidote formula. Some of the 
angles vary quite considerably from those given by Kokscharow—pos- 
sibly due in one or two cases to the striated character of the faces. 
The optical properties are unusual. The axis of greatest optical 
elasticity lies in the obtuse angle 2, making an angle of 1° 51’ to 2° 47’ 
with the vertical axis, according to the nature of the light used. The 
optical sign is positive—an unusual thing for epidote. The indices are 
x = 1.714, 2 = 1.716, and y = 1.724. The double refraction is thus 
-010, the least value known for the mineral. The optical angle over 
a, 90° 32’, is exceptionally large. A comparison of the data at hand 
seems to show that with increasing percentage of iron the double refrac- 
tion becomes stronger, the index of refraction increases, while the opti- 
cal angle (over a) grows larger, and when it passes 90° the crystals 
become optically negative. 
Miscellaneous Notes.—Leiss® gives details concerning several 
new models of optical instruments as manufactured by Fuess of 
Steglitz, near Berlin. The most important of these are a petrographi- 
cal microscope, a theodolite-goniometer, an optical angle instrument, 
and a number of devices for universal motion—vViola® shows the 
application of the quaternion method to the discussion of crystal sym- 
metry, and arrives at results concordant with those of Fedorow, Schön- 
flies and others.—Schwarzmann’ describes a scale for reading directly 
with approximate accuracy the apparent optical angle 2E, without the 
labor of calculating it by Mallard’s formula—Crystallographers will 
be much interested in the results obtained by Rinne’ in certain experi- 
5 Neues Jahrb., B.B. X, pp. 179-195; also pp. 412-439, 1895. 
ë Neues Jahrb., B.B. X, pp. 495-532, 1896. 
7 Neues Jahrb., 1896, Vol. I, pp: 52-56. 
8 Neues Jahrb., 1896, Vol. I, pp. 139-148. 
