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" On the Application of Maxwell's Curves to Three-colour Work, with Especial Reference to the Nature of the Inks to be employed, and to the Determination of the Suitable Light- filters." By REGINALD S. CLAY, B.A., D.Sc. Communicated l.y Sir W. DE W. ABNEV, K.C.B., F.R.S. Received April 25, -Read June 20, 1901.
PART I. THEORETICAL.
1. Three-mlour Projection.
Maxwell showed that any colour in the spectrum could be matched by a suitable mixture of three monochromatic lights, red, green, and violet, and his celebrated curves give at each part of the spectrum the intensity of these lights necessary to imitate the colour at that point both in hue and luminosity. Other experimenters have since repeated his measurements with improved apparatus ; and throughout this essay I shall employ the curves found by Sir Win. Abney as being probably the most accurate.!
Thus it is possible to photograph a spectrum in three colours only. Three negatives must be taken through " niters " which alloAV the colours to pass respectively in amounts determined by the above curves. One filter must allow light to pass according to the red curve, i.e., the extreme red is just transmitted, and the bright red fully passed. Then the filter absorbs the yellow slightly, and the absorption increases along the spectrum, until in the blue and violet it is nearly complete. From this negative a transparency is made, and projected with red (monochromatic) light on a white screen. So for the green and violet. Assuming this done, we have a spectrum illuminated everywhere with the three monochromatic lights in the proportions indicated by the curves. Thus the spectrum will be reproduced exactly. (See Notes 1 and 2.)
If, instead of illuminating the three transparencies with mono- chromatic lights, we use red, green, and violet lights produced by passing white light through coloured glasses (providing that these lights are of the same dominant hue as the primary colour sensations), we shall still obtain a spectrum that appears almost correct. The light obtained by a coloured glass is not monochromatic, but, if the hue is
t " The Colour Sensations in Terms of Luminosity," ' Phil. Trans.,' 1899.
