(a) The colour of an abnormal beam may be masked to the vision from excess of luminosity.
(b) The luminous intensity of the abnormal beam may be too low to excite definite colour sensations.
7. The vision has a varying rate of appreciation for different colours by time, the lowest being for red. The rate increases in rapidity through the spectrum, until the maximum rate is reached with violet. And since this varying rate necessitates a time limit for critical observations, five seconds has been adopted as the limit, no variations being perceptible in that time.
Laws 8 and 9 relate to Colour Constants.
8. The colour of a given substance of a given thickness is constant so long as the substance itself, and the conditions of observation, remain unaltered.
9. Every definite substance has its own specific rate of colour development for regularly increasing thicknesses.
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.