The isosceles-triangular prism is a crystal, mounted in brass and held by boxwood columns fixed on a support with a joint that allows it to be suitably oriented.
The prism has the property of deflecting light rays that arrive from a source by angles that depend on their colour. When sunlight passes through a prism it separates into its differently coloured components: red, orange, yellow, green, blue, indigo and violet – the colours of the rainbow.

Instrument on display.