The respective positions of the primaries of each theory in regard to the whole cycle of distinguishable colours are illustrated above, and the primaries of each theory are shown in their several spectrum positions, the spectrum being shown as bent in circular form. The six principal theories of primary colours are given in Table I, and illustrated on Plate I, with the names of the primaries of each theory opposite the names of some of their principal advocates. It should not be forgotten when comparing these wide divergences, that each theory has been the result of experimental evidence, in what was, at the time, and remains up to the present, a new and progressive branch of science. They agree that the spectrum colours are purer than the pigmentary colours, and that by reason of their being referable to wave length positions, they are most adaptable as standards of colour. There has also been common agreement that certain colours are primaries, and that all other colours are mixtures of these, but there has been wide divergence as to their number and even the colours themselves. [Illustration: PLATE I ILLUSTRATING THE POSITION OF THE PRIMARIES IN RELATION TO THE COLOURS REQUIRED TO BE PRODUCED BY THE THEORETICAL MIXTURES AND ALSO IN RELATION TO THE CHROMATIC CIRCLE. A SEVEN RAY THEORY. NEWTON. A SIX RAY THEORY. WERNER. A FIVE RAY THEORY. NEWTON (_Early_). HELMHOLTZ (_Early_). A FOUR RAY THEORY. HERING. A THREE RAY THEORY. CHEVREUL. BREWSTER. HAY. REDGRAVE. FIELD. A THREE RAY THEORY. YOUNG-HELMHOLTZ. _To face page 4._] [_Lovibond, Colour Theories._ ] CHAPTER II. Evolution of the Method. The writer was formerly a brewer, and this work had its origin in an observation that the finest flavour in beer was always associated with a colour technically called “golden amber,” and that, as the flavour deteriorated, so the colour assumed a reddish hue. It was these variations in tint that suggested the idea of colour standards as a reliable means of reference. The first experiments were made with coloured liquids in test tubes of equal diameter, and by these means some useful information was obtained; but as the liquids soon changed colour, frequent renewals were necessary, and there was always a difficulty and uncertainty in their exact reproduction.