A luminous ring of spectral colours is conjured by a simple, yet intriguing apparatus, whose mechanism is on display to viewers. A spotlight shines through a series of concentric holes in three metal discs and onto a round prism. A fourth metal disc blocks the light dispersed by the prism, casting a shadow on the opposite wall and focusing the projected light into a narrow ring of rainbow colours. The order of the colours, however, is the opposite of those normally presented by the rainbow, with the cooler tones on the outside and the warmer ones on the inside.