26 Dr. R. S. Clay. On the Application of 



(11.) Brans, H. "Beitrage zur Entwicklung der Muskulatur und des peripheren 

 Nervensystems der Selachier," ' Morphol. Jahrb.,' 1899. 



(1.2.) Punnett, B. C., "On the Formation of the Pelvic Plexus, &c., in the Genus 

 Mustelus," < Phil. Trans.,' B, vol. 192, 1900. 



" 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. 



