116 Brush and Dana—Danburite from Russell, N. Y. 
The axial ratios for the two species are :— 
~ 
(vert b 
Danburite 0°8830 1°8367 1:0000 
Topaz 0°9024 1:8920 10000 
The above values show that the two species are closely homceo- 
morphous. 
peter ry to measure both of them in oil. By this measure- 
ment the interesting result was reached that the acute bisectrix 
for the lower end of the spectrum (red and yellow rays) is 
normal to the brachypinacoid, and for the Oppet end of the 
spectrum (blue rays) normal to the macropinacoid. 
rom a section cut parallel to the ehstiy pinacktdl the follow- 
ing angles were obtained, each being the mean of a large num- 
ber of measurements : 
Red (Lithia flame). Yellow (Sodium flame). Blue (CuSOx solution). 
100° 33” 101° 30” 104° 36” 
From a section parallel to the — oo the angles ob- 
tained in the same manner were 
Red (Li). Yellow (Na). Blne (CuSQ4). 
106° 35” 05° 36 02° 13” 
From these angles the true — axial angle was calcula- 
ted by the usual method; the res 
Bisectrix normal to } (010). Bisectrix normal to a (100). 
Red (Li) 87° 37” 92° 23” 
Yellow (Na) 88° 23” 91° i 
Blue (CuSO,) 90° 56’ 89° 
The cs pul mae to the ee ee} is series and 
that normal to the macropinacoid is positiv 
The index of refraction of the oil pared was found to be 
for 
Red (Li) = 14706; Yellow (Na) = 1°4735; Blue (CuSO,) = 1-483 
For obvious reasons the last value is less accurate than the 
other two. Making use of these values in the usual formulas, 
the mean index of we (8) for ae is obtained, viz: 
B= 1-634, Red (Li 
<7 oer Yellow tifa) 
= 1°646, Blue (CuSO,) 
It is obvious from the values of the axial angles for the differ- 
ent colors given above, that for certain rays, those falling in 
