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.
It is more instructive if we experiment with pure colours, and so we must resort to our colour patch apparatus described in [Fig. 6]. If a small circular aperture about quarter of an inch in diameter be cut in a card, and placed in front of the prism nearest the camera lens (Fig. 32), the colour patch, instead of being an image of the face of the prism, will be an image of the circular hole, and when the slit is passed through the spectrum we shall have a coloured spot on the screen, on which we can superpose a patch of white light from the reflected beam. There are two ways in which we can reduce the intensity of the spot, by narrowing the slit through which the spectral ray passes or by placing the rotating sectors in front of the coloured beam. This last, perhaps, is the readiest plan, as it only involves the reading of the sector. We can then diminish the intensity of the coloured spot to such a degree that by its dilution with white light it will entirely disappear. It will be found that red disappears at a different aperture of sector to that required for the green, and the green to that for the blue.
From our previous experiments in chapter VII. we know the luminosity of the spectrum to the eye, and it will be of interest to see what relation the luminosity at which the spots of different colour disappear, when they are so diluted with white light, bear to the total luminosity of these rays.
In a set of measurements made it was found that the reduced angular apertures required for the colours indicated by the following were:
| B | required | 300°* | of aperture. |
| C | " | 56° | " |
| D | " | 14° | " |
| E | " | 22° | " |
| F | " | 150° | " |
| G | " | 2100°* | " |
The large numbers marked with an asterisk were obtained by placing the rotating sectors in front of the white reflected beam.