7. When a light colour is juxtaposed to a dark colour, the light colour appears lighter than it is, and the dark colour darker.[10]
8. When colours are juxtaposed, they become influenced as to their hue. Thus, when red and green are placed side by side, the red appears redder than it actually is, and the green greener; and when blue and black are juxtaposed, the blue manifests but little alteration, while the black assumes an orange tint or becomes "rusty."
9. No one colour can be viewed by the eye without another being created. Thus, if red is viewed, the eye creates for itself green, and this green is cast upon whatever is near. If it views green, red is in like manner created and cast upon adjacent objects; thus, if red and green are juxtaposed, each creates the other in the eye, and the red created by the green is cast upon the red, and the green created by the red is cast upon the green; and the red and the green become improved by being juxtaposed. The eye also demands the presence of the three primary colours, either in their purity or in combination and if these are not present, whatever is deficient will be created in the eye, and this induced colour will be cast upon whatever is near. Thus, when we view blue, orange, which is a mixture of red and yellow, is created in the eye, and this colour is cast upon whatever is near; if black is in juxtaposition with the blue, this orange is cast upon it, and gives to it an orange tint, thus causing it to look "rusty."
10. In like manner, if we look upon red, green is formed in the eye, and is cast upon adjacent colours; or, if we look upon yellow, purple is formed.
HARMONY.
11. Harmony results from an agreeable contrast.
12. Colours which perfectly harmonise improve one another to the utmost.
13. In order to perfect harmony, the three colours are necessary, either in their purity or in combination.
14. Red and green combine to yield a harmony. Red is a primary colour, and green, which is a secondary colour, consists of blue and yellow—the other two primary colours. Blue and orange also produce a harmony, and yellow and purple, for in each ease the three primary colours are present.
15. It has been found that the primary colours in perfect purity produce exact harmonies in the proportions of eight parts of blue, 5 of red, and 3 of yellow; that the secondary colours harmonise in the proportions of 13 of purple, 11 of green, and 8 of orange; and that the tertiary colours harmonise in the proportions of olive 24, russet 21, and citrine 19.