In the same way we find that the colour sensation of (18) is made up of the sensations of (17) and (28).
(18) = 4(17) + 1/10(28).
In this way all the different colour sensations can be referred to the sensations which we may happen to consider as best representing the fundamental sensations. What these are is a matter still unsettled; though from the equations formed by colour-blind people, who only require really two colours to form equations, their places are approximately known; evidently as before said, the ray in the spectrum which the green colour-blind person sees as white light, is that where to the normal eye the green fundamental sensation is purest, being free from predominance of either of the other two sensations, and might be taken as a standard colour. Now if our luminosity curve is correct, and if the sum of the luminosities of each colour separately is equal to the luminosity of the colours when mixed (which we have shown to be the case in chapter VII.), it follows that the correctness of the measures can be checked by using the widths of the slits as multipliers of the luminosities. These luminosities can then be added together, and they should equal in luminosity the white light with which the comparison was made. The results can be compared together by reducing the equations to the same standard of white light.
The following is a set of observations which bear this out.
The red and violet slits in this case were kept at 35 and 17·8 on the scale, and the position of the green slit altered.
| Position of Slits. | Aperture of Slits. | Luminosity of Colour. | Sum of the Luminosity of Each Colour Multiplied by the Aperture. | ||||||
| R | G | V | R | G | V | R | G | V | |
| 35 | 28·5 | 17·8 | 115 | 38 | 112 | 18·1 | 73 | ·65 | 4930 |
| 35 | 28·0 | 17·8 | 119 | 45 | 100 | 18·1 | 61·5 | ·65 | 4989 |
| 35 | 27·75 | 17·8 | 122 | 52 | 85 | 18·1 | 52 | ·65 | 4960 |
| 35 | 27·35 | 17·8 | 125 | 65 | 74 | 18·1 | 40 | ·65 | 4907 |
| 35 | 27·0 | 17·8 | 128 | 78 | 67 | 18·1 | 33·2 | ·65 | 4954 |
| 35 | 26·3 | 17·8 | 133 | 125 | 40 | 18·1 | 20·3 | ·65 | 4987 |
| 35 | 26·0 | 17·8 | 134 | 150 | 10 | 18·1 | 16·7 | ·65 | 4952 |
| 35 | 25·85 | 17·8 | 135 | 170 | 0 | 18·1 | 15·0 | ·65 | 4993 |
| Mean 4959 | |||||||||
The red slit was at a point in the spectrum between C and the red lithium line, and excited probably the fundamental sensation of red alone. The violet slit was close to G, and probably in this case the fundamental sensation of violet was almost excited alone. With the green slit the reverse was the case, all three fundamental sensations being excited. At 26·3 the green sensation was probably the fundamental sensation mixed with white light alone, as at that point the green blind person saw white light in the spectrum, on the red side of it there being what he describes as a warm colour, and on the violet side a cold colour.
An inspection of the table will show how very closely the sum of the luminosities agree amongst themselves, the white light formed by them in each case being of equal intensities. It must be recollected that white light is not necessary to form colour equations; colours may be mixed to form any other colour, which may be taken as a standard. This is often useful in the case of the light between the violet and the blue, where the luminosities are small compared with the luminosity in the green, yellow, and red.
Fig. 35.—Kœnig's Curves of Colour Sensations.