T II E 

 LONDON, EDINBURQH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



JUNE 1883. 



LV. Colour-Sensation. By H. R. Droop, M.A* 



THE generally received theory of colour-sensation is that 

 there are three colour-senses in the eye, and that all the 

 different impressions of colour received by the brain are due 

 to those three colour-senses being affected in different propor- 

 tions by the light entering the eye. This theory was origi- 

 nally propounded by Young, and was revived by Helmholtz 

 and Maxwell, who established experimentally certain laws of 

 colour-sensation, from which laws the theory of three colour- 

 sensations was a probable, but (as I shall show further on) not 

 a necessary, inference. 



Maxwell proved experimentally f that a linear equation of 

 the form 



X = vY + cC + u~U 

 could always be found, expressing any colour and shade of 

 colour perceived by the normally constituted eye in the terms 

 of any three given colours (whether pigments — e. g. vermilion, 

 chrome yellow, and ultramarine — or selected rays of the spec- 

 trum) as seen by the same eye. This equation, when the 

 coefficients v, c, and u are all positive (i. e. when the three given 

 colours are sufficiently intense and distinct from each other), 

 means that the colour X could be produced by combining 

 together (i. e. presenting to the eye simultaneously) certain 

 proportions of the three given colours. From this it obviously 



* Communicated by the Physical Society ; read April 28, 1883. 

 t See Transactions of the Eoyal Society of Edinburgh, vol. xxi. ; 

 Philosophical Transactions, 1860, p. 57. 



Phil. Mag. S. 5. Vol. 15. No. 96. June 1883. 2 E 



