It is now a thoroughly well-established fact that a satisfactory match to any coloured light observable in nature can be made by a combination, in suitable proportions, of three standard lights suitably chosen from a spectrum. It is customary to choose for the standards a red, a green, and a blue light of definite average wave-length in each case. The greatest deviation from the average is made so slight that there is no visible difference in colour between the extreme components present in any one standard. A simple way (used by Maxwell) of obtaining these standards would be by means of an ordinary spectroscopic arrangement in which light is focussed on a slit, and is parallelised by a collimating lens, after which it passes through a prism and the object glass of a telescope, on whose focal plane a spectrum is thus formed.