Ultraviolet

Ultraviolet is not a colour the eye can see: it is electromagnetic radiation of shorter wavelength than violet, discovered by Johann Wilhelm Ritter in 1801 when he found that invisible rays beyond the violet end of the spectrum darkened silver salts faster than visible light. Some animals, including many insects and birds, do perceive ultraviolet, and flowers carry ultraviolet patterns invisible to us that guide pollinators.

Because it lies just past the highest visible colour, ultraviolet functions symbolically in modern esoteric systems as the band above the crown, beyond the ordinary spectrum of the seven chakras, standing for the transpersonal or the spiritual reaches past individual consciousness. This usage is entirely modern, a product of twentieth-century aura and subtle-body literature that extended the visible rainbow of the chakra scheme into the invisible bands at either end. As a physical fact ultraviolet is simply high-energy light; the meanings attached to it are recent and analogical, built on its position beyond sight.

Ultraviolet is not a colour in the sense that the rest of this list are colours. It is a band of electromagnetic radiation beyond the violet end of the visible spectrum, and no human eye sees it directly.

It was found by inference. In 1800 William Herschel put a thermometer beyond the red end of a dispersed spectrum and recorded a temperature rise, demonstrating radiation past the visible. The following year Johann Wilhelm Ritter looked at the other end, using silver chloride, which darkens on exposure to light. The salt darkened faster beyond the violet than anywhere in the visible band. Ritter had no thermometer reading to offer and called what he had found chemical rays, a name that fits the evidence better than the modern one does.

What follows from that is a genuine limit on colour symbolism. Every tradition described in the atlas assigns meanings to colours on the basis of what people can see, and ultraviolet was outside that entirely until two centuries ago. Where a modern source presents ultraviolet as an ancient correspondence, the attribution is being made backwards.

The band is not, however, universally invisible. Many insects, including honeybees, see into the near ultraviolet, and a substantial number of flowers carry ultraviolet patterns that are invisible to us and function as guides for pollinators. Some birds and fish see it too. The world has been marked in a colour humans do not have for far longer than humans have existed, and photographic evidence of it dates only from the twentieth century.