The colours developed by means of these scales are governed by the same law of selective absorption which governs the development of natural colours, any of which can be matched and reproduced by means of their established ray proportions. [Illustration: Fig. 7.] The governing law is simple, and may be stated by saying that the colour developed is always complementary to the colour absorbed, not in the generally accepted sense that their mixture necessarily makes white light, but in the sense that they are opposite in the cycle of daylight colours. The dimensions of the unit are necessarily arbitrary, it was originally selected as being a convenient depth for distinguishing differences, the scale was then constructed by equal additions and sub-divisions; the two essentials of a scientific scale being complied with, in that the divisions were equal and the unit recoverable. The power of recovery lies in the fact that different parts of the scale are co-related to physical colour constants, which can be prepared in any laboratory. [Illustration: PLATE VIII ABSORPTION CURVES OF SIX DYES. _To face page 76._] [_Lovibond, Colour Theories._ ] _Specific Colour._--The relationship of colour increase to intensity increase in substances has hitherto been somewhat obscure. It has been sometimes considered that they were in direct proportion, but in the absence of a means of recording colour sensations, no definite results were obtainable. [Illustration: Fig. 8.] Sufficient information is now available to warrant the formulation of the following law: “That every substance has its own rate of colour development for regularly increasing intensities, which, when once established, becomes a constant for identifying similar substances in future.” This is the meaning of specific colour, and when a series of measurements at regularly increasing densities of a given substance have been made, the specific colour rate of that substance is established. This can be charted in curves and used as a basis for estimating quantities, properties, changes of condition, differences in value, detecting adulteration, etc. _Applications._--The author has permission to use the names of several gentlemen who have used the tintometrical scales for various purposes. Sir Arthur H. Church, F.R.S., has employed the tintometrical standards for the purpose of registering the colours of certain wild flowers. Sir Boverton Redwood has used the scales and system for petroleum investigations. At his instance the specific colour rate of petroleum was established, and the several composing colours plotted in curves, as in Fig. 8, where the ordinates represent the scale of units irrespective of colour, and the abscissæ the scale of strata thicknesses.