Abstract

When a source of light is small, such as the points of an arc light, a candle, or lamp, it is comparatively easy to find the luminosity of any coloured surface which is illuminated by it, using the method which has been described in “Colour Photometry, Part II ”; but when the source of light is a large surface, such as the sky, the method therein described is much more difficult to apply. Quite recently, when examining the question of providing suitable screens for producing the negatives required for three-colour photographic prints, it became necessary to devise a plan by which rings of different colours could be made of equal luminosity in ordinary daylight by rotating them with the proper proportions of black.

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