The above experiments have a great bearing on the theory of colour vision, and should be considered very carefully in connection with the shortened spectrum which we have shown exists when red colour-blind people are observing its luminosity. There is one point to be recollected in relation to the mixtures of the three or two different colours which make white light. If different coloured pigments be illuminated by the "made" white light, they will not appear of the same hues, as a rule, as when viewed by ordinary white light. They will vary not only in colour, but in brightness. This might be expected when the spectral light which they reflect is taken into account. CHAPTER X. Extinction of Colour by White Light--Extinction of White Light by Colour. In the last chapter we have shown the impossibility of matching the hue of the simple colours between the violet and the green, unless a certain and appreciable quantity of white light be added to them. We will now turn to a phase of colour measurement which will materially help us to see why, in some cases, the addition of white light to the simple spectrum colours, between the red and green, does not appear necessary in order to make a match with a mixture of red and green. We will ask ourselves two questions: one is, whether any colour, and if so how much, can be added to white without appearing to the eye? and the other, if any, and if so how much, white light can be added to a colour without its being perceived? Perhaps one of the readiest methods of explaining exactly what we mean is by a rotating disc. Suppose we have a red disc, of nine or ten inches in diameter, and at every one inch from the centre paste on it a white wafer about one-eighth of an inch in diameter, and cause it to rapidly rotate. On examination we shall find that pink rings will be formed by the combination of the white and red near the centre, but that towards the margins no rings will be visible, owing of course to more red being combined with the same amount of white. This shows that the eye is only sensitive to a certain degree, and cannot distinguish a very small diminution in colour purity. The intensity of the light has something to do with the number of these pink rings which are visible, as may readily be tested in a room. If the rotating disc be placed near a window, and the number of rings visible be counted, a different number will be visible when it is placed in a dark corner. A kindred experiment is to place red circular wafers upon a white disc, and note the rings visible. This gives the sensitiveness of the eye for the diminution in intensity at the other end of the scale. It will be found that there is a marked difference between the two. Fig. 32.--Diaphragm in front of Prism.