On the luminosity curves of persons having normal and abnormal colour vision

1. When reading a recent paper before the Royal Society, and also in the Press, Dr. Edridge Green has stated that he can find no connection between the luminosity and the colour sense of persons having either normal or abnormal colour sensations. Since I feel that to allow such a statement to go unchallenged might be interpreted as meaning that no such connection could be shown to exist, I propose in the following paper to place before the Society some of the evidence which indicated that there is in reality a very intimate relation between luminosity and colour sense. The results given include a small part of those which have been obtained in a series of experiments which have occupied the last two years and form part of investigation which is still in progress. The term “luminosity” as used in this paper has the following meaning: Suppose that light from some source, such as the electric arc, is admitted to a spectroscope by means of which a real pure spectrum is produced, and that a slide in the plane in which the spectrum is formed carries a slit of fixed width. Light of sensibly one wave-length,i. e. monochromatic light, will pass through this slit, and by means of a lens placed in the beam of this light an image of the first face of the prism which is used to from the spectrum can be formed on a screen. In this way a monochromatic patch of light is obtained, the brightness of which depends on the nature of the source of light, the width of the collimator slit, the width of the slit placed in the spectrum, which for short will be called the movable slit, and the dimensions of the lenses employed. Further, if alongside this coloured patch is formed a white patch of light produced by light which proceeds from the same source but has not undergone dispersion, and that by some means or other the intensity of this white light is altered till the coloured and white light, measured in any arbitrary units, measure the luminosity of the light of that colour which is passing through the movable slit. Since the unit in which the white light is measured is arbitrary, we are not concerned with the absolute intensity of illumination of the white patch, and may use any device we like to alter the quantity of white light which falls on the screen so long as we are able to measure the ratio of the quantity of light employed in different experiments. It will further be observed that for any given person the measurement of the luminosity of a given coloured light in the spectrum involves the comparsion of the brightness of the coloured patch as it appears to him with the brightness of the white patch as it appears to him.