The information to be obtained by charting measured colour is more extensive than appears at first sight, as by varying the character of the co-ordinates, and charting suitable series of measurements, new fields of investigation are opened, thus throwing light on some hitherto obscure questions, of which the following are some instances. SPECIFIC COLOUR. It has sometimes been assumed that colour increase was in direct ratio to intensity increase, but this is never the case, each substance has its own rate, specific to itself. It is conceivable that the colours of two substances may coincide at one point, but as their densities increase, or decrease, their rates of change vary. The term “Specific Colour” is based on the experimental fact, that the colour of a given substance is constant, so long as the substance itself and the conditions of observation, remain unaltered. During experimental work a sufficient number of instances have accumulated to warrant the writer in advancing and using the term “Specific Colour” as describing a new natural law, as rigid in its application as that of “Specific Gravity” or “Specific Heat.” [Illustration: PLATE V ABSORPTION CURVES OF FIVE COLOUR CONSTANTS. _To face page 33._] [_Lovibond, Colour Theories._ ] When this principle is applied to the measurement of regularly increasing thicknesses, curves of colour changes can be established, which are specific for the substance in question, and afford a certain means of identifying similar substances in future. This is effected by varying the nature of the co-ordinates, making the ordinates to represent the tintometrical scale of colour units irrespective of colour, whilst the abscissae represent the scale of increasing thicknesses. Then by plotting the separate factors of each measurement according to their unit values, a series of curves is established, specific to the substance in question, and applicable to none other. We have now two systems of charting colour, in the first, the complete sensation is represented by a single point, as in Plate IV. In the second, each factor is represented by a separate point, and by connecting points of similar colours, a series of curves is established which represents a quantitative analysis of the progressive colour development, as in Plate V. CHAPTER X. Representations of Colour in Space of Three Dimensions. The relations of the different colours to one another, and to neutral tint are, perhaps, best represented to the mind by a solid model, or by reference to three co-ordinate axes, as employed in solid geometry (_see_ Fig. 2). [Illustration: Fig. 2.]