The equation of a colour matched by 17·0 red and 2·6 blue units, is as follows:-- Standard Glasses. Colour Developed. Red. Blue. Violet. Red. 17·0 + 2·6 = 2·6 + 14·4 The colour developed is a red violet in these proportions. A colour matched by Standard Glasses. Colour Developed. Red. Yellow. Orange. Red. 10·0 + 3·0 = 3·0 + 7·0 The colour developed is a red orange in these proportions. A colour matched by Standard Glasses. Colour Developed. Yellow. Blue. Green. Yellow. 3·0 + 1·5 = 1·5 + 1·5 The colour developed is a yellow green in these proportions. A colour matched by Standard Glasses. Colour Developed. Blue. Red. Blue. Violet. 6·0 + 1·8 = 4·2 + 1·8 The colour developed is a blue violet in these proportions. The standard glass colours are necessarily of a given brightness, and colours for measurement may be either brighter, or sadder than the standards. A given complex colour of less than glass standard brightness, is matchable by unequal numbers from the three standard scales; the smallest unit value always represents the “black,” or neutral unit factor. The equation is as follows:-- A colour matched by Standard Glasses. Colour Developed. Red. Yellow. Blue. Neutral Tint. Green. Blue. 1·0 + 3·0 + 9·0 = 1·0 + 2·0 + 6·0 The colour is a blue green, in the proportion of six to two, saddened by one of neutral tint. A given complex colour of greater brightness than the glass standards, is first dulled by the interception of neutral tint units, until measurable in the manner described above; the intercepting glasses represent the unit value of excess of brightness, and is shown in the equation as light units, for instance-- Standard Glasses. Colour Developed. Neutral Tint. Yellow. Blue. Light. Green. Yellow. 1·5 + 7·5 + 0·5 = 1·5 + 0·5 + 7·0 The colour is a yellow green in the proportions of 7·0 of yellow, to 0·5 of green, and 1·5 brighter than the standards. Every daylight colour being thus measurable by a suitable combination of standard glasses, with or without the addition of a Light, or a Neutral Tint factor, it follows that any colour can be described both qualitatively, and quantitatively, in terms of the colour sensations yielded by the standard glasses and their combination. The distinct colour sensations are those, which, by common consent are known as Red, Yellow, Blue, Orange, Green and Violet, and they are yielded by single glasses, or by pairs as already described; all colours therefore fall into the following categories:-- _A._--Single colour sensations:-- 1. Transmitted by single glass standards: Red. Yellow. Blue. 2. Transmitted by equivalent pairs of standard glasses: