46 PROGRESS OF MICROSCOPICAL SCIENCE. 
also displaying the play of colours of the spectrum. From among 
them he had selected several which he exhibited to the Academy as 
illustrating in an unusually distinct manner the structure of the pre- 
cious opal. One of the specimens is white opal, emitting on one side 
from the free surface a brilliant display of colours. These are 
reflected from facets ranging from } to 1 mm. in breadth, and of 
irregular polyhedral form. The facets are distinctly separated by 
fissures, which, in the polishing of the stone, have become more or less 
filled with dirt, and they appear to form the surface of a mosaic pave- 
ment laid on a basis of amorphous opal, of which the other side of the 
specimen consists. The facets are distinctly striate; the strie being 
parallel on the same facet, but changing in direction on the different 
ones, though pursuing the same general course over comparatively 
large areas, as represented in the same figure. The striz, or tubes as 
Sir David Brewster considered them to be, vary in degree of fineness ; 
some apparently being double or more the thickness of others. He 
had not attempted to measure them accurately, but they appear to be 
about the size of the lines in the ordinary micrometer eye-piece of the 
microscope. There appeared usually to be about four or five striz in 
the space of ;4, of a mm. Another specimen is a dark carnelian- 
hued fire-opal, which exhibits in directly looking upon it, just beneath 
the surface, a patch of round or oval spots of a deeper hue. The spots 
range from 1 to 1 mm. in breadth, and are separated by interspaces 
from 1 to 2 of a mm. The spots appear as lenticular disks with 
finely striated surfaces; the striz being parallel, and on the different 
spots pursuing nearly the same course. Viewed at a certain angle, 
they mainly emit a rich golden-green hue. In another opaque white 
specimen, emitting rich hues, the striated facets are more or less 
isolated by amorphous opal, and vary much in size. The smaller 
facets are generally irregularly oval; the larger ones appear to be 
made up of an aggregate of the smaller kind. Over comparatively 
large areas, the striw of the different facets pursue nearly the same 
direction, but in contiguous areas they even pursue quite opposite 
directions. On the larger patches, also, as I have attempted to repre- 
sent them, the strie are not perfectly continuous, but appear to be 
rather interrupted in bands. On another part of the same opal the 
brilliant patches would appear to pertain to cylindroid or fusiform 
rods of the striated opal imbedded in amorphous opal. The striz in 
these rods appear to be arranged in regular parallel layers, so that 
either longitudinal or transverse sections give rise to the appearance 
of parallel striz. From these specimens precious opals would appear 
to be constituted of an aggregation of particles of a striated or finely 
tubular structure which may be imbedded in a basis of more amor- 
phous opal. When isolated by the latter, the particles may appear 
as lenticular disks, round or oval balls, or cylindrical rods with 
rounded ends and of variable length. When closely aggregated these 
particles become more or less polyhedral. The particles in section 
in any direction present a striated appearance, and, according to the 
varying fineness of the strie, and their inclination, emit the varied 
hues for which the precious opal is so much admired. 
