The theory of the Scientists is based on the phenomena developed by mixing coloured lights taken from different parts of the spectrum. This is a method of synthesis, each added colour being a progressive stage towards the complexity of white light. In this case the colour developed is that of the preponderating ray of a complex beam. The theory of the Artists is based on the phenomena developed by mixed pigments. This is a method of analysis, tending towards ray simplicity, each added pigment reducing the complexity of the colour developed by its power of selective absorption. The theoretical differences between the two schools appear to have arisen from supposing that a given colour developed by the two methods should correspond; but considering the differences in their ray composition, this would be impossible, for although both may be describable by one general colour term, as for instance a Red, they would be of two varieties. It remains to be shown that one theory may cover both sets of phenomena. The Red, Green, and Violet theory appears to be based on two principal assumptions: first that there are only three fundamental colours; and second that the rays taken from different colour areas are pure colours. Both assumptions are open to question. In regard to the first, there is no difficulty in isolating six colours; and as to the second, it can be demonstrated that the colours do overlap in every part, with a double overlapping in the middle colours, and are therefore not simple but complex. PAST THEORIES. In a work of this nature it is unnecessary to deal minutely with the theories which have been adopted from time to time since Newton’s discovery of the continuous spectrum. It will, however, be useful to touch on the principal points where theorists are agreed, and also on some of their points of difference, the latter in order to find, if possible, the causes of their difference. TABLE I. ----------------+-----+------------------------------------------------- | No. | | of | Primary Colours. |Rays.| ----------------+-----+------------------------------------------------- Newton (later) | 7 | Red, Orange, Yellow, Green, Blue, Indigo, Violet | | Werner | 6 | Red, Orange, Yellow, Green, Blue, Violet | | Newton and | | Helmholtz | 5 | Red, Yellow, Green, Blue, Violet (early) | | | | Hering | 4 | Red, Yellow, Green, Blue | | Chevieul, | | Brewster, Hay, | | Redgrave, Field | 3 | Red, Yellow, Blue | | Young, Helmholtz| | (later) | 3 | Red, Green, Violet ----------------+-----+------------------------------------------------- Note to Plate I.