The luminosities of the following colours were taken by the above method, and subsequently we shall have to use their values. Electric Light. White 100 Vermilion 36 Emerald Green 30 Ultramarine 4·4 Orange 39·1 Black 3·4 Black (different surface) 5·1 Suppose we have two or more colours of the spectrum whose luminosities have been found, the question immediately arises, as to whether, when these two colours are combined, the luminosity of the compound colour is the sum of the luminosities of each separately. Thus suppose we have a slide with two slits placed in the spectrum, and form a colour patch of the mixture of the two colours and measure its luminosity, and then measure the luminosity of the patch first when one slit is covered up, and then the other. Will the sum of the two latter luminosities be equal to the measure of the luminosity of the compounded colour patch? One would naturally assume that it would, but the physicist is bound not to make any assumptions which are not capable of proof; and the truth or otherwise is perfectly easy to ascertain, by employing the method of measurement last indicated. Let us get our answer from such an experiment. +-------------+---------------+ | Colours | Observed | | Measured. | Luminosity. | +-------------+---------------+ | R | 203·0 | | G | 38·5 | | V | 8·5 | | (R + G) | 242 | | (G + V) | 45 | | (R + V) | 214 | | (R + G + V) | 250 | +-------------+---------------+ Three apertures were employed, one in the red, another in the green, and the third in the violet, and the luminosity was taken of each separately, next two together, and then all three combined, with the results given above. The accuracy of the measurements will perhaps be best shown by adding the single colours together, the pairs and the single colours, and comparing these values with that obtained when the three colours were combined. When the pairs are shown they will be placed in brackets; thus (R + G) means that the luminosity of the compound colour made by red and green are being considered. R + G + V = 250·0 (R + G) + V = 250·5 (R + V) + G = 252·5 (G + V) + R = 248·0 (R + G + V) = 250·0