Additivity in the tetrachromatic colour matching system.

The trichromatic system of measurement. although suitable for fields subtending around 2 . does not give a linear metric with larger fields because of the presence of rod receptors. Their presence is allowed for in the tetrachromatic colour match and so one might expect the tetracllromatic system to be more linear. it is the purpose of this paper to test this by compariq tetra~hromatie and trichromatic additivity for a variety of test stimuli. A special case of the additivit? principle. luminance metamerism. presents a much more severe test than the general case: while the latter is only touched on. the former is concentrated on. Most experiments involve measurement hut some employ subjective judgement. No departure from tetrachromatic additivity has yet been detected. althoucgh trlchromatic nonadditivity is pronounced. Trichromatic additivity failures so far examined are always in a direction consistent with the hypothesis that “rod colour” is blue. An indication is given of the way in which the additive property of the tetrachromatic colour match can be utilized to develop general systems of calorimetry and photometry. applicable at all luminance levels.