Natural Ph i losophy . — Optics . 217 
luminous point always appears in the same librationof the moon, 
the opinion of Dr Gibers will become highly probable. 
OPTICS. 
4. Optical Properties of Euclase.— In the summer of 1820, 
Mr Brooke was so kind as to send me a fine specimen of this 
rare mineral for optical examination. I found it to have Two 
axes of double refraction, as I had predicted in 1817 from its 
primitive form, ( Phil . Trans. 1818, p. 225.) The principal 
axis, which is Positive , is coincident with no line in the primitive 
form of the crystal, as is the case in Sulphate of ' Lime , Ky unite , 
and Tincal, and the neutral axes do not coincide (as happens in 
kyanite and tincal) with the axes of the rhomboidal prism in 
which it crystallises. This property, so uncommon in crystal- 
lised bodies, indicates some peculiarity in the structure of these 
minerals, of which crystallographers are not aware. — D. B. 
5. A new Primitive Form detected in Boracite.— In a paper 
recently laid before the Royal Society of Edinburgh, Dr 
Brewster has shewn that Boracite possesses a new primitive 
form, which has not been recognised by crystallographers, viz. 
the Rhombohedral Cube , or the Cube with one axis , which forms 
the limit between the acute and the obtuse rhomboids. This 
result was deduced from the fact, that Boracite has One positive 
axis of double refraction, coincident with one of the diagonals of 
the solid, or the axis of the rhomboidal cube. 
6. Distribution of the Colouring Matter in Topaz.— In a se- 
ries of experiments on the distribution of the colouring matter 
in topaz, laid before the Royal Society of Edinburgh at its last 
meeting, and forming a continuation of the experiments given in 
the Phil. Trans. 1819, p. 11. Dr Brewster has shewn, that in 
several specimens of the Yellow Brazil Topaz, the inclination of 
the resultant axes is in general about 50° 5', and sometimes less, 
whereas in the Blue Topaz of Aberdeenshire, and the Colour- 
less Topaz of New Holland, it is about 65°. But what is very 
remarkable, the one resultant axis is inclined only 22° 3V to the 
axis of the prism, while the other is inclined 27° 28' ; so that the 
principal axis of double refraction in the Yellow Brazil Topaz 
is not perpendicular to the surface of the laminse. Specimens 
of these different minerals have been sent for analysis to an emi- 
nent foreign chemist, in the confidence that they are different 
substances, differing probably in the quantity of Fluoric Acid. 
