If we suppose the divisions of the circle to commence from A, the boundary of the red and violet, we shall have the following Table, which will enable us to determine the resulting colour without the aid of a diagram.
As the construction of the preceding Table is very obvious, from the inspection of Fig. 4., a single exam ple will be sufficient to explain its use. Let it be re quired, therefore, to determine the colour which results from a mixture of all the colours, except blur. In the third column, we find, that the point in, opposite to the centre of gravity of the blue arch, is in the 330th degree of the circle, which appears, from the fourth column, to lie between the limits of the red arch, viz. 315 to 360: therefore the resulting colour will be red, but with a small mixture of orange, as the 330th degree, or the point in, is between the centres of gravity of the red and that of the orange arches, being 7' from the former, and from the latter.
With the aid of these preliminary observations, we are in a state of preparation for explaining the pheno mena of accidental colours. When the eye is fixed Liu some time upon a red square, the part of the retina, which receives the image of the square, is strongly ex cited by the continued action of the red rays. The sen sibility of that relaxed portion of the retina to red light, must therefore be diminished, in the same way as the palate, when accustomed to a particular taste, ceases to feel its impression. But if the red rays, which after wards fall upon the relaxed part of the retina, are feeble compared with those which issued from the red square, and produced the relaxation; or if the taste, which is afterwards presented to the palate, is much weaker than that which first diminished its sensibility, then it is still more obvious, that the debilitated portion of the respec tive organs will not be susceptible of these feebler ex citements.
When the eye therefore is turned from the red square to the white paper, the enfeebled portion of the retina is excited by the white light which flows from the pa per, but is not sensible to the impression of the red rays which enter into the composition of this white light.
The debilitated part of the retina, therefore, is excited by all the component colours of white light, except the red, or by the colour resulting from their combination. But it will be found, from the preceding Table, that this resulting colour is blue with a mixture of green, or bluish-green, consequently the relaxed part of the reties will be sensible only to this colour, and will perceive a bluish-green square upon the uhite paper, of the same size as the red square, if the white paper and red square were held at the same distance ; but of a greater or less size, according as the distance of the white paper is greater or less than the distance of the red square. Hence the accidental colour of red is bluish-green, or, in general, the accidental colour of any natural colour is that which results from the mixture of all the colours of the spectrum, except the natural colour itself. When the square first viewed by the eye is black, it is obvious that the part of the retina on which its image falls, is not excited by any rays, while all the surrounding part of the membrane is excited, and enfeebled, by the image of the white paper upon which the bhtck square is placed. If the eye, therefore, be fixed upon a white ground, the light of this ground will make the strongest impression upon the unexcited part of the retina, and, consequently, there will appear, on the white ground, a square whiter than the surrounding portion. The very reverse of this will happen, when a white square, upon a black ground, is viewed by the eye.
From this hypothesis we may now construct a table of accidental colours more accurate than that IS hick Buffon deduced from experiment. The table, however, which is thus formed, is founded on the supposition, that the natural colours employed arc of the same kind as the prismatic ones.
Natural Colours. Accidental Colours.