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 



