242 Transactions of the Royal Society of South Africa. 
of birefringence of the mineral, no colours are produced. The cross remains 
stationary within a comparatively wide range of motion of the section ; on 
the rotation of the latter it breaks up into two hyperbolae — but this separa- 
tion, although apparent, is so slight that we hesitate to proclaim the 
biaxial nature of the figure. We do, however, regard it as being a true 
axial figure. 
The sign of the mineral, determined from these figures by means of a 
quarter undulation mica plate, is negative. 
The Opalescent or slightly Clouded Hyalite. 
Thin sections of this variety have a very faint brownish tint when viewed 
by transmitted light. The important difference between this and the 
clear hyalite, so far as optical behaviour is concerned, is in the sign of the 
double refraction ; whereas the nature of the double refraction of the clear 
is negative, that of the slightly clouded is positive. In other respects, what 
has been written concerning the clear applies also to this variety. 
Compound Spheroids. 
Mention has been made of the occurrence of spheriods of the slightly 
clouded hyalite having an external layer of colourless hyalite. A section 
intermediate between a tangential and a diametral section was prepared 
from one of these. When this section was examined in plane polarised light 
it exhibited an extinction figure composed of a black cross and a black ring 
— the latter being comparatively close to the margin of the section. On 
testing this figure with a quarter undulation mica plate it became apparent 
that the black ring was of the nature of a compensation band resulting 
from the overlapping of the two varieties — positive and negative — conse- 
quent on the curvature of the spheroid. 
Subsequently a section was made through a cluster of such spheroids, 
which, on examination, revealed the fact that in some spheroids there is 
a repetition of concentric layers of positive and negative material. 
Further Experiments and Observations. 
(a) Examination by means of X-rays. 
X-rays were passed through a thick diametral section of the clear 
hyalite, and the effect of the mineral on the rays was ascertained by means 
of a photographic plate — after the method of von Laue. The " pattern " 
showed that, for a given radius, the dispersion had been uniform around 
the central spot. 
