+------------+-------------------------------------------+ | | | 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 × 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.