1. Normal white light is made up of the six colour rays, Red, Orange, Yellow, Green, Blue and Violet in equal proportions. When these rays are in unequal proportions the light is abnormal and coloured. 2. The particular colour of an abnormal beam is that of the one preponderating ray, if the colour be simple, or of the two preponderating rays if the colour be complex. The depth of colour is in proportion to the preponderance. 3. The rays of a direct light are in a different condition to the same rays after diffusion, and give rise to a different set of colour phenomena. Laws 4, 5, 6, and 7 deal with The Limitations of the Vision to appreciate Colour. 4. The vision is not simultaneously sensitive to more than two colours in the same beam of light. The colour of any other abnormal ray is merged in the luminosity of the beam. 5. The two colours to which the vision is simultaneously sensitive are always adjacent in their spectrum order, Red and Violet being considered adjacent for this purpose. 6. The vision is unable to appreciate colour in an abnormal beam outside certain limits, from two causes: (_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. 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.