Dr. North on the Angle of Aperture in Microscopes. 221 
be written (}R*+3) Sic. Moreover, oligoclase has similarly the 
oxygen ratio of Hornblende = (4R*+38) Sif, Hence we may 
look upon Leucite, and Andesine, with Pyroxene; as in a certain 
sense trimorphous. Still, their relation to the feldspar series is 
such that they are naturally classed with the other feldspars. 
The zeolites, if the water be excluded, have the oxygen ratios 
of the feldspar-section, as shown in the following table; the oxy- 
gen of the water in the zeolites is annexed to the name of the 
Species :— 
Oxygen ratio.  Feldspars. Zeolites. 
RE Bi 
1:3:4 Sodalite, Anorthite, Ittnerite (2), Thomsonite (24). 
1:38:44 Nepheline, - Zeagonite 4 
1:3:6 Labradorite, Levyne (4), Natrolite (2), Scolecite (8). . 
1:3:8 Leucite, Andesine, Analcime (2), Chabazite (6 or 5). 
Philippsite (5), Laumonite (4). 
P3879 Oligoclase, Harmotome (5), Chabazite (6). 
£e3:12 Orthoclase, Albite, Heulandite (5), Brewsterite (5). 
: Stilbite (6 or 5), Epistilbite (5). 
Some of the species are correspondingly isomorphous with 
feldspar species, as Analcime with Leucite, Ittnerite with So- 
dalite; and the ratio 1: 3:12 produces oblique forms in both 
series. But we do not intend to draw a general parallelism, as 
the water whatever its relations, must in some cases modify the 
ratios. But as regards the origin of the species, the table is an 
interesting one. Bischof remarks on the identity in the ratio be- 
tween the oxygen of the. bases and silica in chabazite and that 
of Hornblende, and thereby explains the occurrence of pseudo- 
morphs of chabazite after hornblende. 
Pyrrhotine (Fe7S*) and Greenockite (Cd S).—As Pyrrhotine 
and Greenockite are homeomorphous, they are naturally arranged 
in the same group, although the former has a little too much 
sulphur. The formula 5Fe S+F'e? S*, may perhaps be written 
FeS [+1Fe? S*], the latter term being unessential to the type. 
— 
Art. XXI1.—On Microscopes with large Angles of Aperture ; 
by Dr. E. D. Norru. 
si y ; 
“5m the whole diameter of the front lens receives a pencil of rays 
tom each minntest point of the object, and that, consequently, 
€n these pencils from each point are large, more light is re- 
re oe e points separately as well as from the entire object, 
