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.
In more detail
Where the stone's slow ray lines up with the slow direction of the plate, the color climbs to blue. Where they cross, it falls to yellow.
How we measure it
Our Polariscope's red plate is a 550 nm linear retarder, with its slow direction from northeast to southwest. The page reads the sign from the figure's interference colors alone.
Related terms
- 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.
- Conoscope: 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.
- 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.
- 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.