615. The optical product of the juxtaposition of contrasted flat colours is composed of two effects:—1. The effect which arises from each of the juxtaposed colours, by receiving the complementary of the colour contiguous to it, is thus strengthened or tinged agreeably, independently of any augmentation of gloss. 2. The effect arising from gloss in the two juxtaposed colours. Here it may be remarked that associations which I have not prescribed, such as red with violet, or blue with violet, have a fine effect in the plumage of certain birds and butterflies; for, in these natural associations, the effect arising from the addition of the complementaries to each, which would injure the _flat_ colours, is entirely insensible in surfaces which acquire _metallic brilliancy_ from their organic structure. Finally, I shall add, that it would be necessary, before raising the objection, to demonstrate that the same red, associated with green, the same violet associated with yellow, and the same blue associated with orange, equally glossy, would be less effective than in natural assortments. _Influence of Form taken into consideration in the Effect of Contrast of two Colours._ 616. Elegance of form, the arrangement of the parts, their symmetry, the effects of light and shade, and the association of ideas which may connect this form with an agreeable recollection, will prevent the perception of the ill effect of two associated colours, even when not glossy. Thus, for example, in flowers, combinations, which would not produce a good effect upon two plain surfaces, are very beautiful. For example, the flower of the sweet pea, which has the combination of red and violet. 617. The critic must be directed by the considerations here summed up:— 1. The kind of association: the greater the difference between the colours, the more they beautify each other; and, the less difference there is, the more they will tend to injure one another. 2. The equality in depth of tone. 3. The difference of tone, the one being deep, the other light. 4. The glossiness of the surfaces which reflect the colours to the eye. 5. The form of the coloured body. OF THE COMPLEX ASSOCIATIONS OF COLOURS, REVIEWED CRITICALLY.