A conoscope is a strain-free glass sphere set on a stone in a polariscope. It gathers light from many directions at once, so the eyepiece shows an interference figure.
In more detail
With the figure, a gemologist can tell a uniaxial stone from a biaxial one. With a first-order red plate, the figure also gives the optic sign.
How we measure it
Our Polariscope's conoscope maps a 40° cone of directions in air. The 40° is a working choice, not a measured sphere.
Related terms
- Interference figure: An interference figure is the pattern of colors and dark bands that a doubly refractive stone shows through a conoscope. Each point of the figure is one direction through the stone.
- First-order red plate: A first-order red plate is a retarder in a polariscope that shows which of a stone's two rays is slower. It turns parts of the interference figure blue or yellow, and that gives the optic sign.
- Optic sign: The optic sign of a uniaxial stone tells which of its two rays is slower. It is positive when the extraordinary index is higher than the ordinary one, and negative when it is lower.
- Polariscope: A polariscope is a gem-testing instrument with a stone between two polarizing filters. With the filters crossed, it shows whether a stone is singly refractive, doubly refractive, strained or an aggregate.