Further investigation on these lines has placed the violet of the spectrum as a primary rather than the blue, but this is still a matter of debate. Suffice it to say that a red and a green in the spectrum are really two of the primary colours, and most probably the violet the third. Experiment shows that there is no other primary colour in the strict sense of the word. We thus arrive at the fact that, except the primary colours themselves, every colour in nature may be made by a mixture of two or three of these primaries. Just a word of explanation as to why, with pigments, the primary colours appear to be red, yellow, and blue, and not red, green, and blue. The colour of a pigment, it must be recollected, is a complex one. If we analyse a yellow--a yellow glass will be just as good an example as anything else--we find it is made up of green, yellow, orange, and red. A blue is made up of blue and green. If a yellow is placed behind a blue glass, and we look at a white surface through them, the only light that can get through the glass is the green. If the light, coming through each glass _separately_, falls on the same spot on a white surface, it will be either colourless or bluish white, or yellowish white, whichever colour preponderates. As the light reflected from mixed pigments is made up principally by the light coming through the different particles, first coming through one and then through another, and only partially by mixed lights, it will be gathered why the primary colour, when deduced from experiments with pigments, was yellow, and not green. With the spectrum colours there is this fact to remember, that though all intermediate colours between the pairs of primaries can be formed by their mixture, yet in some cases the resulting colours are _slightly_ diluted with white, and that they thus appear less saturated than the spectrum colours themselves. The reason for this we shall be able to account for when we consider the colour sensations themselves.