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1834. succession. With a little attention it will generally be found that, whenever the eye is affected by one prevailing colour, it sees at the same time the accidental colour, in the same manner as in music the ear is sensible at once to the fundamental note and its harmonic sounds. The imagination has a powerful influence on our optical impressions, and has been known to revive the images of highly luminous objects months and even years afterwards.'

The general reader of this passage, as well as of M. Plateau's original memoir, can scarcely fail to be struck with the extraordinary announcement that blackness arises from the union of all the colours of the spectrum, when they are the accidental colours and the discovery must appear to him an important one. A slight examination of the subject, however, will show us that this proposition is a verbal illusion; and that the physical fact which it so erroneously expresses, has been long known to philosophers.

An accidental colour is something essentially distinct from a colour produced by the action of direct rays. The rays which produce ordinary colours, can be combined in any proportion we please; and the resulting effect is the sum of the actions of each separate ray upon the retina. Hence all the different colours of the spectrum produce a purely white beam of light; and perfect whiteness may also be produced by two compound colours, one of which is complementary to the other. An accidental colour, however, cannot be added to, or combined with, another. When the eye sees an accidental colour, suppose red, the excited part of the retina is insensible to all other rays but those of the accidental colour. If we instantly excite the same portion of the retina with another light which is an accidental green, and thus render it insensible to red, then the eye will see blackness, not because the accidental red and the accidental green compose blackness, but because the eye has been in succession rendered insensible to the two colours which compose white light itself. If Buffon, or Dr Darwin, or Count Rumford, had been asked what would be the effect of exciting the retina in quick succession with all the simple colours of the spectrum, or with two compound colours which compose white light, they would all have immediately answered, blackness. M. Plateau has therefore, in this part of the

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