Let the three adjacent edges OR, OB, OY, of the above cube be three axes, along which are measured degrees of Red, Yellow and Blue respectively, starting from the origin O. Every point in space on the positive side of this origin will then represent a conceivable colour, the constituents of which in degrees of red, yellow and blue are measured by the three co-ordinates of the points. Pure reds lie all along the axis OR, pure yellows on the axis OY, and pure blues on the axis OB. All normal oranges, normal greens, and normal violets lie on the diagonals of the faces of the cubes OO^1, OG, OV respectively. Pure neutral tints lie on the diagonal ON of the cube, equally inclined to the three principal axes. Red violets will be found on the plane ROB, between OV and OR. Blue violets on the same plane between OV and OB. “Saddened” red violets all within the wedge or open space enclosed by the three planes, whose boundaries are OB, OV, ON. The other colours, red and yellow oranges, blue and yellow greens, pure and saddened, are found in corresponding positions in relation to the other cases.[4] [4] This method of illustration was suggested by Dr. Herbert Munro. CHAPTER XI. The Spectrum in relation to Colour Standardization. The spectrum has naturally been considered as a suitable source for colour standards, but the power of analysing has disclosed some difficulties, which have yet to be overcome. Concerning the prismatic spectrum, there has always been a difficulty in apportioning the different colours to specific areas, and further, before this spectrum is available for colour standardization, some method of correction for the unequal distribution of colours must be devised. Neither of these difficulties occur in the use of the diffraction spectrum, where the pure colours are apportioned by Professor Rood from A to H in the manner shown in table on next page. Professor Rood further divides the spectrum from A to H into 100 equal divisions, allotting 20 unit divisions of 72,716 wave lengths to the space between each two colour lines. This allots a space of 3,635 W.L. to each unit division, as shown in Table III. TABLE III.