_Quartz_ belongs to the uniaxial system of crystals, and accordingly exhibits one series of rings only, and no perfect central black cross.
On revolving the tourmaline the colour gradually changes, and passes through all the colours of the spectrum. It can be cut to exhibit either right-handed polarisation or left-handed polarisation and also to exhibit straight lines.
_Calc Spar._--A uniaxial crystal showing only one system of rings, and a black cross, changing into a white cross on revolving the tourmaline.
_Topaz._--A biaxial crystal exhibiting only one system of rings with one fringe, owing to the wide separation of the axes. The fringe and colours change on revolving the tourmaline.
_Borax._--A biaxial crystal; the colours are seen to be more intense than in topaz, but the rings not so complete--only one set of rings can be seen, owing to their wide separation.
_Rochelle Salt._--A biaxial crystal; the colours are more widely spread out than the former, and only one set of rings seen at the same time.
_Carbonate of Lead._--A biaxial crystal; axes not so far separated, and both systems of rings are more widely spread than those of potassium nitrate.
_Aragonite._--A biaxial crystal; axes widely separated, but both systems of rings seen at the same time. A fine crystal for displaying the biaxial system.
[Illustration: Fig. 185.--Crystal of Potassium Nitrate.]
It was long believed that all crystals had only one axis of double refraction; but Brewster found that the greater number of crystals which occur in the mineral kingdom have _two axes_ of double refraction, or rather axes around which double refraction takes place; in the axes themselves there is no double refraction.
Potassium nitrate crystallises in six-sided prisms with angles of about 120°. It has two axes of double refraction. These axes are each inclined about 2-1/2° to the axes of the prism, and 5° to each other. If, therefore, a small piece be split off a prism of potassium nitrate with a knife driven by a sharp blow of a hammer, and the two surfaces polished perpendicular to the axes of the prism, so as to leave the thickness of the sixth or eighth of an inch, and then a ray of polarised light be transmitted along the axes of the prism, the double system of rings will be clearly visible.
