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.

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.

The light of D had to be reduced to 14° before it was extinguished; therefore to extinguish the original light of this colour in the spectrum would require 180/14, or 12·9 times the intensity of the white light of the reflected beam. With the E light it would take 180/22, or 8·2 times the white light to extinguish it, and so on. If we tabulate the results in this manner, and take the white light necessary to extinguish the D light empirically as 98·5, which is its percentage luminosity in the spectrum of the electric light, we can then compare the extinguishing factor with the luminosity in each case.

+------------+-------------------------------------------+ | | | White required| | | |White required| to extinguish | Luminosity | | Colour. | to Extinguish| the Spectrum, | of | | | the Spectrum.|with 50 as That| Spectrum. | | | | required at E.| | |------------+--------------+---------------+------------+ |near line B | ·6 | 3·9 | 4·9 | | C | 3·2 | 19·5 | 20·6 | | D | 12·9 | 78 | 98·5 | | E | 8·2 | 50 | 50 | | F | 1·2 | 7·5 | 7·5 | | G | ·087 | ·56 | ·6 | +--------------------------------------------------------+

The very close resemblance between the last two columns indicates that the same luminosity of white light is necessary to extinguish the same luminosity of most colours, within the limits of observation that is to say. Indeed the method of extinction was a plan which Draper and Vierordt essayed, but the results, tabulated from experiments made by them with the apparatus they employed, give a curve of intensity very unlike that given in Chapter VII. In these experiments the luminosity of the orange light corresponding to the D line coming through the slit was measured, and it was found to be 37·5/180 of the white light. Now according to the last table but one 14/180 of this light was extinguished by the full white light, consequently 37·5/180 x 14/180, or 1/62 of the orange light was extinguished by the white light. In other words, if white light be sixty-two times brighter than the orange light, the colour of the latter when the two are mixed will be invisible. The extinction of all colours requires somewhat more light than this, and a calculation shows that the extinction of every colour is effected by white light, which is seventy-five times brighter than the colour. Artists are well aware that a pale wash of a pigment may be washed over drawing paper, and when dry is invisible to the eye. The above experiments fully account for it.

The other experiment which was to be tried was to see how much white light could be extinguished by a colour. There are several ways by which this can be effected. For instance we may superpose a white dot on the colour patch by placing a card, in which a circular hole is cut, in the reflected beam near the prism, from which the reflection takes place; or by putting a black circular disc of small dimensions pasted on a glass in the same position, by which means the white light is superposed over the whole of the colour patch, with the exception of what, when the colour is cut off, is a black spot; or again by placing a rod to shade half the patch from the white light, but leaving the whole of it exposed to the coloured beam. All these methods have been tried, and it appears that the size of the piece of the patch over which the white light is thrown may have some effect on the resulting curve, but of one thing there is evidence, viz. that a great deal more white light can be mixed unperceived with orange light, than can be with the green, blue, or violet. From one experiment it was found that 1/36 part of white light of the same luminosity as the orange could be mixed with the orange and not be perceived; but that with the green light at E 1/90 would just be visible, whilst at F in the blue-green the 1/120 could be distinguished. Looking at these results, and applying them in elucidating the experiments in which it was attempted, but without success, to match the intermediate colours between violet and green (of which the light at F is a case in point), by mixing them together, unless white light were added to the simple colour; and the success of the other experiment, in which orange light could be obtained of the same hue as that at D by a mixture of the red and green, it will be noticed that 3·3 times more white light can be added to the orange than to the green light at F, without its perception. The white light produced by the mixture in the first case might well show when mixed with the green, but might pass wholly unperceived when mixed with the orange.