595. The phenomena of simultaneous contrast being less evident in a very vivid light than in a weaker light (63), it follows that if we disregarded the difference in the effects, we should greatly deceive ourselves in our appreciation of the phenomena of contrast of similar colours. Simultaneous contrast, which tends to make the differently coloured parts appear as distinct as possible, is carried to a maximum, precisely when the light being feeble, the eye requires the greatest contrast of colour to perceive distinctly the various parts upon which it is fixed. 596. We may perceive the modifications presented by bodies when lighted, and yet we may experience much difficulty in accounting for them, for want of knowing how to represent exactly the modifications which the coloured materials experience, in their colour, according as they receive light or white, shade or black; or according as they are mixed together. It is partly to make these modifications clearly known that I have designed the _chromatic hemisphere_ (134, _et seq._). In describing it, I have attached less importance to its material realization than to the rational principle upon which it depends. On looking at the lines of this diagram independently of all colouring, we understand how any colour is reduced by white, deepened by black, and broken by black and white, and how, by mixture with a pure colour, it produces hues. I shall add subsequently some new considerations on the gradations of colour made with coloured materials. My object would not have been attained had not the chromatic hemisphere given me the means of representing, by a simple nomenclature, the modifications which a colour undergoes by the addition of white and black, modifications which produce the _tones of its scale_; those which it receives from black yielding _broken scales_, and those resulting from the addition of a pure colour, produce scales which are _hues_ of the first colour. 597. Finally, to the definitions which I have given of the words _tone, scale, hue, broken colours_, I must add the distinction of the associations of colours into _harmonies of analogy_, and _harmonies of contrast_ (172). I am convinced that all those who accept the small number of definitions I have given, will find much advantage from them in accounting for the effects of colours, and in expressing their views to others. By their aid it will be easy to notice relations which might have escaped observation, or which, in the absence of precise language, could not have been clearly communicated. 598. It would be ignoring a fact to attribute the opinion I have combated, exclusively to ignorance of what I have just recapitulated, or to believe, that in order to establish the contrary opinion, which I maintain, it is sufficient to dissipate this ignorance. But I am satisfied with pointing out the error, without making the least pretension to overthrow it, otherwise than by stating what I believe to be the truth.