4 
654 The American Naturalist. [July, 
Thus it is shown that, in this case at least, crystals do not grow by 
the deposition of parallel layers of substance, but that new faces are 
constantly being developed which obey the law of rational (though 
not simple) indices. Their mutual inclinations in the case of alum 
show that the face to which they approximate is always the octahe- 
dron with angle 70° 31%’, hence the faces of this form do not vary 
their inclination as supposed. 
Determination of the Principal Indices of Refraction for 
the most Important Rock-making Minerals.—Zimanyi‘ has 
determined by the method of total reflection (using a modified Kohl- 
rausch total reflectometer) the principal indices of refraction, and 
hence, at the same time, the double refraction, of the more important 
rock-making minerals. He has found that methylene iodide, which 
has not before been used with the total reflectometer, is a particularly 
good enclosing medium, since it was found to suffer scarcely any 
change in the course of an entire year. His paper gives the results of 
a very extensive series of determinations on no less than fifty-five spe- 
cies or varieties, A few of the determined values are given below: 
Mineral. Mean index of refr. Doublerefr. Opt. Char. 
Albite (Schmirn). 1.5337 > 0.0105 ot 
Elsolite (Laurvik). 1.5350 0.0042 — 
Nepheline (Vesuvius). 1.5407 0.0050 — 
Orthoclase. 1.5222 0.0064 — 
Sodalite (Ditro) 1.4834 
Nosean (Laach). 1.4950 
Hauyne (Latium). 1.5027 
Leucite (Vesuvius). 1.5086 
Cordierite (Bodenmais). 1.5396 0.0091 — 
Muscovite (Buckfield). 1.5861 0.0388 i 
Augite (Pojana). 1.7000 0.0250 —? 
Biotite (Diff. localities). 1.5600-1.5894 
Tremolite (Diff. localities). 1.6117-1.6135 0.0252-0.0270 — 
Actinolite (Diff. localities). 1.6150-1.6257 0.0271-0.0280 _ 
Tourmaline (Diff. loc.) 1.6324-1.6357 0.0184-0.0239 
Amphibole (Kafveltorp), 1.6463 0.0163 + 
Sillimanite (Saybrook). 1.6641 0.0200 +- 
Olivine. 1.6710 0.0359 ES 
Zoisite (Tyrol). 1.7010 0.0050 + 
t Zeitsch. f. Kryst., XXII, pp. 321-358 (1894). 
