426 jp Scientific Intelligence. 
a Sabe a square, ab; a crystal is seen upon it secured with wax, 
t its centre is seen the orifice for the passage of the light, its 
size sorpusyaes tae to that of the crystal to be examined : a bene 
point on the edge is made a notch, which is adapted to a pin 
to the inside of the tube so that the position of the aia pupa “shall 
remain fixed. By these means, the crystal under examination may be 
properly adjusted two sides of the engraved square must coincide 
with the axis of the analyzing tourmaline, while two are at right angles 
with it. The movable cylinder carries an index, fig. 1, z, which trav- 
erses the graduations of ‘the graduated semicircle. 
If now a cleavage plate of calcite be so fixed upon the diaphragm 
that a’ b’ is parallel with a d of the rigid equare “89 in fig. 3, an 
the movable cylinder be adjusted to zero, then a’ J is also parallel with 
the longitudinal axis of the tourmaline, and no black cross is visible. 
The cylinder must now be turned until ya cross appears normal, and 
the angle of revolution may be read off on the graduation 
' T have investigated various crystals in thts manner, and ‘have arrived 
at the following results which in general characterize the systems of 
esd on ead 
Hexaconat System.—Through parallel hate of the hexagonal 
‘pyram: mid, the vertical arm of the black cross is seen at right angles to 
the basal edge (edge Z). Through parallel planes of the rhombohedron 
the cross arranges itself in the direction of vig diagonals, Throug 
the planes of the prism, the vertical arm of the black cross stands in 
the direction of the ie axis. Examples, quartz, calcite, beryl, 
‘soda nitre, corundum, apat 
Seen through the basal agli the cross is not changed by a pévolit- 
tion of the cylinder. (Tourmaline has peculiarities, the ac ecount of 
which is Wisi omitted. 
Dimeric etinehe. —Through parallel planes of the square pyramid, 
the same relation was observed as in the hexagonal system: the same 
is true also for the planes of the prism, and the basal planes. 
When the edges of the pyramid are laid horizontal the cross appears 
revolved. Ex. meionite, vesuvian, apophyllite, zircon, mellite, chalco- 
lite, rutile, etc. 
Trimeteic System.—Seen through two parallel planes of the rhom- 
bic pyramid the normal arm of the cross is. not at right angles with 
the basal edge. Ex. sulphate of zinc, of magnesia, and of nickel, 
topaz,* &c. * 
Seen through the basal planes of the rhombic prism, the cross takes 
the direction of the diagonals. Ex. epsomite, topaz, muscovite, ara- 
gonite, barytes, &c. 
Through the lateral faces of the vertical rhombic and’ seers 
Bae the cross stands in the direction of the vertical axis. 
