+--------+-----------+----------+-----------+-----------+ | | Sunlight. | Electric | Gaslight. | Skylight. | |--------+-----------+----------+-----------+-----------+ | Red | 100 | 100 | 100 | 100 | | Green | 193 | 203 | 95 | 256 | | Violet | 228 | 250 | 27 | 760 | +--------+-----------+----------+-----------+-----------+ Now from the above it might seem that as three simple spectrum colours will give us the colour of any pigment, that therefore two colours ought to give us the same colour as any intermediate simple colours in the spectrum which lie between them; for instance, that the simple blue-green ought to be obtained by mixing spectral green and spectral violet together. This can be ascertained with a single colour patch apparatus, by cutting a slit in the card that fills up the aperture between the two adjustable slits, and deflecting the beam transmitted through it by a right-angled prism, and back on to the screen through another similar prism, as described in chapter VIII. It is more convenient, however, to use a duplicate apparatus precisely similar to the first, with the exception that no collimator is required, placing them side by side, and mirrors making the reflected beam from the first traverse the second set of prisms. There will be a reflected beam from the second apparatus, which can be utilized in the same way as was that from the first apparatus, and the two spectra will vary together in brightness, as will also the new reflected beam, since they all are formed by the light coming through one slit. A patch of the colour intermediate between the two is thrown on the screen from the second apparatus, and the second patch from the first apparatus overlaps it. A rod placed in the usual manner throws two shadows, which are illuminated by the two different beams. If blue-green be a colour it is wished to match, it will be found that no matter in what part of the violet and green the slits are placed, no match can be effected. But if some very small quantity of red light be mixed with simple blue-green, that then a colour identical in every respect as regards the eye can be obtained from the violet and green of the first apparatus. It must be remembered that a mixture of red, green and violet form white, and that they are mixed in definite proportions. No matter how feeble in intensity the white may be, the same proportions will still obtain. In the above experiment, as the blue-green must contain violet and green, the small quantity of red must combine with the proper proportion of violet and green, and will form white light, so that the match is obtained by the residues of the violet and green mixed with the small quantity of white light, of which the red is the indicator.