[3] It was found that the superimposition of two glasses did not visually disturb equivalence, therefore only two glasses were used for each observation in constructing the scales. The yellow and blue scales, were similarly constructed, taking care that their similar unit values were in colour equivalence with the red units, the test of equivalence being, that when equal units of the three scales were superimposed against a normal white light, a neutral grey was transmitted, in which no trace of colour could be perceived by the common consent of the whole staff of trained observers. The scales were then considered as in colour equivalence with each other. The system of cross-checking was so elaborate, that after the equivalence of the first unit was established, nearly four years was occupied in the work before the scales were passed as satisfactory. It may be urged that the unit is arbitrary, but this applies also to the unit of any other standard scale; it is sufficient that the essentials of a philosophic scale are complied with, in that the divisions are equal, and the unit recoverable. [Illustration: PLATE IV COMPARISON CURVES OF HEALTHY HUMAN BLOOD WITH THE BLOOD OF LOWER ANIMALS. _To face page 31._] [_Lovibond, Colour Theories._ ] CHAPTER IX. Colour Charts. A colour chart is constructed by placing two colour scales at right angles to each other, with their zeros at the angle. A measured simple colour, finds its position directly on its corresponding colour scale at the point of its measured value. A measured complex colour, finds its position within the angle, at that point where perpendiculars drawn through the two colour values meet. The above statements are complete only for colours of standard brightness, should the colour be brighter or duller than standards, a light factor is necessary, the value of which is furnished by the measurement itself, and must be written in numerals near the colour point. By this method the chart position of even the most complicated colour is indicated by a single point which is determined by the analytical value of the composing factors. EXAMPLES. Simple Colour of | Complex Colour Standard | of Standard Brightness. | Brightness. | 3· Red. | 6· Blue, 10· Violet. Simple Colour | Complex Colour Brighter than | Duller than Standards. | Standards. | 7· Yellow, | Red 6, Orange 5, Light 2· | Black 2. The number of complex colour charts is limited to the six represented in Fig. 1 as lying in their order on a continuous spectrum. The red and violet mixtures having no visible spectrum position are represented in the ultra violet. The ordinates of the charts are made by erecting the overlying red, yellow and blue scales as perpendiculars. [Illustration: Fig. 1.]