CHAPTER III.

Evolution of the Unit.

The dimensions of the light and colour unit here adopted, together with the scales of division, were in the first instance physiological, depending entirely on the skill of normal visions for exactitude. The co-relation of equal values in the different colour scales, was secured by an elaborate system of cross-checking, rendered necessary because the establishment of a perfectly colourless neutral tint unit, demanded an exact balance in values of the different colour scales. These scales have stood the test of many years’ work by many observers, and in no case has any alteration been required. The original set is still in use.

The first point which required consideration after the want of standard colour scales was realized, was the basis and dimensions of the unit. So far as the writer knew there was no published information bearing on this which could be used as a guide.

Several arbitrary scales for specific purposes had already been constructed by selecting a colour depth which could easily be distinguished, calling it a unit, and scaling it by duplicating and subdividing. This course was adopted with a coloured glass which approximately matched Ales and Malt solutions, and another which matched Nesslerized Ammonia solutions. No insuperable difficulty occurred in constructing scales available for quantitative work in these two instances.

The intensity of the colour unit for these arbitrary scales, was that which appeared to be most convenient for the purpose required, but the several scales had no common basis. The unit was physiological, and the exactitude of the scales depended entirely on the skill of the vision for discriminating small differences.

As the writer’s experimental work progressed, it became evident that red, yellow, and blue were the only colours suitable for systematic work. The superimposition of any two, developed a third colour which apparently had no relation to either. The superimposition of the third glass modified or destroyed all colour and reduced the amount of light. This suggested the idea that if the three colours could be so balanced that the light transmitted was colourless, it would be evidence of equivalence of intensity in the individual colours.

The real difficulty was in obtaining this equivalence, because a balance which transmitted a neutral tint by one light developed colour by another. This necessitated the selection of a standard light. The light finally selected was that of a so-called sea fog, away from the contaminating influence of towns. The white fog of Salisbury Plain was used as being most available. It required two years’ work to establish equivalence in the unit.