A red orange, in which red preponderates by one unit, and is one unit brighter than standard, is first dulled by one red, yellow and blue, and then matched by four red, and three yellow, and is correctly described by R. 1·0 + O. 3·0 + Light 1·0 CHAPTER VIII. The Colour Scales. A normal vision under ordinary conditions, has no hesitation in correctly naming the sensations produced by the triad groups red, yellow and blue, or by the single rays orange, green and violet. It can also correctly describe a complex colour sensation, by naming the two associated colours, such as red orange, yellow orange, blue green, blue violet, etc.; but when called upon to decide differences of colour depth, it can only do so by using arbitrary terms of no precise scientific value, such as light, medium, dark, etc. This deficiency is because the vision has in itself no arrangement for the quantitative definition of colour depth. This want can only be met by co-relating colour sensations, to some physical colour constants. This co-relation has now been effected by a series of glass standard colour scales, which are numerically graded for colour depth, the scales themselves being colour constants by co-relation to percentage solutions, of such coloured chemicals, as copper sulphate, nickel sulphate, potassium permanganate, etc. These substances as well as many others, are always available for checking the constancy of the scales, or for recovering the unit if lost. As already mentioned, the system of taper scales proved to be useless for the purpose, not only because the rate of colour increase was never in proportion to the rate of thickness increase, but also because no two substances are equal in this respect, each having a rate specific to itself. The prismatic spectrum colours were not available for several reasons, first as being unsuitable for critical comparisons under daylight conditions, as being too weak except “in camera”; also they were found to be too crowded for the separation of a working area of monochromatic colour, and some corrections would have been necessary for variation in the refractions of different colour rays. This is more fully dealt with under the heading of The Spectrum in relation to Colour Standardization, page 36. THE EQUIVALENCE OF THE COLOUR SCALES. The method employed for obtaining equality of the unit divisions, and colour equivalence between the different scales was as follows:-- Two slips of red glass in a light shade were made exactly equal in colour, and considered as initial units; these were then superimposed and matched by a single glass, which was then considered as of two units, this and one of the initial units were superimposed, and matched by a single glass of three colour units, and so on, until a progressive red scale was constructed, ranging in intensity from ·01 to 20· units.[3]