In the same manner the luminosity of any colour can be found. Thus that of a purple, or of green, can be ascertained; of the former by using the green disc with either the red or the blue disc, and the latter by the red and blue disc. From this it is apparent that we can check the luminosities derived from other means by this plan. A taking experiment can be made with colour discs to imitate all the colours of the spectrum in their proper order, though diluted more or less by white light. This can be done by rotating V, E, and U together; but in order to get additional luminosity in the yellow, we can use chrome yellow as well. If a disc be made as in the figure (Fig. 44), it will on rotating give a fair imitation of the spectrum, if it be viewed through a slit held in front of the disc. Fig. 44.--Disc arranged to give approximately all the Spectrum Colours. The mixture of colours by means of rotating sectors is one which the artist cannot use for artistic purposes, and it might seem that for him any deductions made from this method are useless; but it is not so. Suppose we take black lines ruled closely together on paper, and examine the surface from such a distance that the lines are no longer distinguishable it will appear of a grey; and if we take the amount of black on the paper and amount of white, and prepare two sectors of black and white, whose angles are in these proportions, and rotate them alongside the ruled surface, it will be found that the grey of one matches the grey of the other. If instead of lines of black and white we have them of light yellow and cobalt blue, a grey is also produced when the surface covered by the blue is to that covered by the yellow in correct proportions, and may be matched by rotating sectors containing merely black and white. Now some artists employ stippling, filling up cross-hatching of one colour with dots of a totally different colour, or they place dots side by side. When seen from the distance at which the picture should be viewed, these various colours blend one into another, and form a tint which is the same as that which would be obtained by rotating these colours together in the proportion in which they cover the ground. Artists, however, generally mix their pigments together on the palette, and the resulting mixtures are often totally unlike those which are obtained by rotating the same colours together, a noteworthy example is that of yellow and blue. By rotation, and when in proper proportion, these two give a white, but when mixed on the palette a green results. What causes this difference? Experimental proof is always the most satisfactory proof, so let us have recourse to the spectrum apparatus to obtain an answer. Let a spectrum be thrown on the screen, and in it place a strip of paper painted with the yellow, and then another with the blue. With the first it will be seen that the blue rays are not reflected, but only the green and yellow and red, taking the