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302 OPTICAL PROJECTION way—between a blue or a green containing nearly half the spectrum, and a pure blue or green. 178. Composite Colours.—The results of compounding colours have not been what are popularly supposed, blue and yellow, for instance, making white and not green. A great many striking experiments may be made in compounding colours, and especially blue and yellow. Holding a blue glass over the slit as in fig. 166, it will be seen that it transmits blue and green ; a yellow glass transmits yellow and green : green therefore remains when both media are superposed, and it appears that the two produce green as the result of succes- sive subtractions by the blue and yellow. But using good blues and yellows separately, in the stages of a bi-unial, and * allowing the discs to overlap, their addition makes white. Glasses which do this can be found, or the blue may be a cell of neutral or slightly acid copper sulphate ; and the yellow of potash bichromate, or picric acid (both fluids highly poison- ous). The copper and bichromate make, on the other hand, a nearly pure green by absorption ; and a drawing in red and yellow chalk illuminated by this light, appears done in black. On the other hand, a cell filled with a solution of copper oxide in strong ammonia, and one of the bichromate of potash, can be so adjusted in strength, or in thickness of solution by wedge-shaped cells, that one transmits nearly all of the spectrum stopped by the other. These will also give a good white when their separate discs are superposed, but when the cells are superposed, stop all the light, and the screen is nearly dark. The same may be done with a cell of potash permanganate, and a green glass selected by trial; and there is in Chance's glasses a shade called ' signal green,' which, when superposed on a full red, also practically stops all the rays. Experiment with the spectral colours shows that blue and yellow are more really ' compound ' colours than violet or green. Neither of the latter can be made by compounding