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.
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.