In order to make the qualitative symbols quantitative it is only necessary to add the numerical unit value to each factor as found by direct experiment.
At an early stage of the investigation, it was found that coloured glass gave better results than coloured solutions, and that Red, Yellow, and Blue, were the only colours suitable for systematic work; it was also found that any colour could be produced by their combination. As already described arbitrary scales were first used in many colours, but were superseded by these three, which, when graded into scales of equivalent value, were found to cover all daylight colours.
Upon this evidence, scales of red, yellow and blue were constructed of glass slips, each scale being all of one colour, with a regular variation of intensity from 0·01 to 20·0 units, equal units of the three scales being in equivalence with each other. The dimensions of the unit are necessarily arbitrary, but the scales comply with the essentials of a scientific standard, in that the divisions are equal, and the unit recoverable. The equality of the unit divisions in the scales, is demonstrated by a system of cross-checking. The test of colour equivalence has already been described on pages 10 and 28.
The power of recovering the unit, is by co-relation to well-known physical colour constants, such as is easily obtained by definite intensities of percentage solutions, of selected pure chemical compounds in distilled water, at standard temperatures. For example, a one per cent. solution of pure crystallized copper sulphate C_{ 2}SO_{ 4}5H_{ 2}O at 60° F. when viewed in the optical instrument in a 1-inch stratum, must be matched by a combination of Yellow 1·58 and Blue 1·55.
The inch of distilled water itself constitutes very little of this colour; the colour of distilled water is remarkably uniform, and might almost be taken as a colour constant, thus: A 2-foot stratum is matched by Yellow 0·1 and Blue 0·34, a 4-foot stratum by Yellow 1·0 and Blue 1·45.
A one per cent. solution of _Nickel Sulphate NiSO_{ 4}7H_{ 2}O, tem._ 60° F. in a 2-inch stratum must be matched by 2·2 Blue and 2·0 Yellow units.
A one per cent. solution of _Potassium Bichromate_ K_{ 2}Cr_{ 2}O, Tem. 60° and in a 2-inch stratum after being dulled by 0·5 neutral tint units must be matched by 34·0 yellow and 9·6 red units.