analysed band, the spectrum, most bent away from the original line of direction of the white light striking the prism, are said to be the most refrangible rays, and consequently are situated in the most refrangible end or part of the spectrum, namely, that farthest from the original direction of the incident white light.  These most refrangible rays are the violet, and we pass on to the least refrangible end, the red, through bluish-violet, blue, bluish-green, green, greenish-yellow, yellow, and orange.  If you placed a prism say in the red part of the spectrum, and caught some of those red rays and allowed them to pass through your prism, and then either looked at the emerging light or let it fall on a white surface, you would find only red light would come through, only red rays.  That light has been once analysed, and it cannot be further broken up.  There is great diversity of shades, but only a limited number of primary impressions.  Of these primary impressions there are only four—­red, yellow, green, and blue, together with white and black.  White is a collective effect, whilst black is the antithesis of white and the very negation of colour.  The first four are called primary colours, for no human eye ever detected in them two different colours, while all of the other colours contain two or more primary colours.  If we mix the following tints of the spectrum, i.e. the following rays of coloured light, we shall produce white light, red and greenish-yellow, orange and Prussian blue, yellow and indigo blue, greenish-yellow and violet.  All those pairs of colours that unite to produce white are termed complementary colours.  That is, one is complementary to the other.  Thus if in white light you suppress any one coloured strip of rays, which, mingled uniformly with all the rest of the spectral rays, produces the white light, then that light no longer remains white, but is tinged with some particular tint.  Whatever colour is thus suppressed, a particular other tint then pervades the residual light, and tinges it.  That tint which thus makes its appearance is the one which, with the colour that was suppressed, gave white light, and the one is complementary to the other.  Thus white can always be compounded of two tints, and these two tints are complementary colours.  But it is important to remark here that I am now speaking of rays of coloured light proceeding to and striking the eye; for a question like this might be asked:  “You say that blue and yellow are complementary colours, and together they produce white, but if we mix a yellow and a blue paint or dye we have as the result a green colour.  How is this?” The cases are entirely different, as I shall proceed to show.  In speaking of the first, the complementary colours, we speak of pure spectral colours, coloured rays of light; in the latter, of pigment or dye colours.  As we shall see, in the first, we have an addition direct of coloured lights producing white; in the latter, the green colour,