Any sight that could be described as yellowish or bluish is called an inorganic chromatic visual sensation. We use words like yellow, blue, gold, cyan, indigo, brown, orange, violet, turquoise, chartreuse, azure, ocher, cerulean, sepia etc. to identify particular visual sensations within the inorganic category. Ewald Hering reports that, “no color is both yellowish and bluish … yellowness and blueness are mutually exclusive.”1Ewald Hering, Outlines of a Theory of the Light Sense, page 49. Translated by Leo M. Hurvich and Dorothea Jameson. Harvard University Press, 1964. Therefore inorganic visual sensations are susceptible of binary description.
The reference experience for these sensations is seeing gold. So to make a binary description of an inorganic chromatic sensation, compare it to gold. Report the result using one of the following algebraic statements. If the two experiences are not comparable, then say that the sensation is not an inorganic chromatic sensation and express this as 
If the sensation is like seeing gold, then say that it is yellow. Express this as 
If not, then say that it is blue and that 
The number 
is called the yellowness. These relations are expressed mathematically as
Reference Constant for Yellowness
The numerical constant associated with the visual sensation of seeing gold is conventionally known as the vacuum electric permittivity. It is symbolized by 
and has an observed value of 
.
| 1 | Ewald Hering, Outlines of a Theory of the Light Sense, page 49. Translated by Leo M. Hurvich and Dorothea Jameson. Harvard University Press, 1964. |
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