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LVJI. On the Construction of a Colour Map, 

 By Walter Baily, M t A* 



[Plate VIII.] 



BY the term Colour Map I mean a diagram each point of 

 which defines by its position some particular colour. 

 Such a colour map was designed by Clerk Maxwell in the 

 form of a triangle, the angles of which were occupied by 

 certain colours, and all other colours were treated as mixtures 

 of these three primary colours, the composition of the mixture 

 for the colour which occupied any particular point in the tri- 

 angle being indicated by the length of the perpendiculars 

 from that point on the sides of the triangle. 



Now trilinear coordinates, although they afford very elegant 

 methods for the solution of certain problems, are by no means 

 so generally useful or so intelligible as the ordinary rectan- 

 gular coordinates ; and the fact that every colour can be 

 defined by means of a spectrum colour and white light sug- 

 gested to me the construction of a colour map with rectangular 

 coordinates, in which measurement in one direction should 

 indicate the wave-length of the spectrum colour employed, 

 and measurement at right angles to it should indicate the 

 quantity of white light employed in defining the colour. 



Let us take a vertical line to represent the spectrum, the 

 lower end giving the red of the spectrum and the upper 

 the violet. The spectrum is supposed to be formed so that 

 equal differences of length measured along the spectrum 

 represent equal differences in the wave-length ; and when the 

 quantity of colour at any point of the spectrum is mentioned, 

 it is intended that a definite small part of the spectrum about 

 that part is to be taken. Now all colours, except the purples, 

 can be formed by adding white light to a spectrum colour. 

 Let the amount of white light required be indicated by a line 

 measured horizontally to the right from the proper point in 

 the spectrum. Then the given colour is indicated by the 

 point at the extremity of that line. Again, every colour 

 except the greens has the following property : viz. that if it is 

 added in the proper quantity to some spectrum colour, white 

 is produced. Let the quantity of white produced be indi- 

 cated by a line drawn from the proper point horizontally to 

 the left. The point at the extremity of this line indicates the 

 given colour. In this way a map is obtained in which every 

 colour has its appropriate position. The greens occur only on 

 the right hand, and the purples only on the left hand, but all 



* Communicated by the Physical Society : read April 8, 1892. 



