MEMORANDA. 43 
a single (uni-axial) system of prismatically coloured rings, 
with a cross in the centre, changing from black to white 
according to the position of the analyser; while the latter 
belonged to the trimetric system, characterised by a double 
(bi-axial) system of rings, grouped like a figure of «; and 
these optical differences would also indicate differences in the 
physical properties of the two fragments, such as their hard- 
ness, specific gravity, cleavage, fracture, &c. Then, again, 
minerals of various chemical constitution belonging to such 
erystailme systems as exhibit bi-axial axes, may be distin- 
guished by determining the angular value between the centres 
of the two systems of rings, the distance between the cen- 
tres of the two loops in the figure of o form being greater 
in some species that in that of others. Thus, the extensive 
series of micas are now arranged into a few groups charac- 
terised by the angular distance between the centres of the 
bi-axial rings. But the most gorgeous effects are produced 
by films of those very cleavable minerals, mica and selenite ; 
for every plate of a given thickness having a definite colour- 
value, we are able to produce an indefinite variety of colour- 
tints. If the film of selenite is uniform in thickness, it 
produces an even tint; if of varying thickness, a magnificent 
assemblage of colours is the result. 
Taking advantage of this fact, the ingenious optician builds 
up designs of various pretensions to artistic beauty, from 
mere stars of varied-colour rays, to dying dolphins, groups 
of flowers, bunches of grapes, and Gothic windows; or, 
