ACCIDENTAL CoLcens, a name given by Buffon to those colours which arise from the continued action of' light upon the retina, in order to distinguish them from those which are produced by the decomposition of white light.
A few of the phenomena of accidental colours were first obseried by Dc la Hire, and our countryman Dr :ru•n ; but we are indebted to Bufl'omprofessor Scherffer, and illpinus, for a complete series of experiments, by which the nature and cause of these colours have been almost completely unfolded. The limits of our work will not permit us to give a detailed view of the various experiments by which this subject has been illustrated ; hut by directing the attention of the reader to the most important facts. and to the theory by which they may he e_plained, :.e will be enabled to account for the various optical illusions which are referable to the same cause.
\\Then we look steadily, and for a considerable time, at a small square of red paper placed upon a white ground, we perceive a light green border surrounding the red square : by removing the eye from the red square, and directing it to another part of the white ground, we perceive very distinctly a square of light-green approach ing a little to blue, and of the same size as the real red square. This imaginary zreen is the accidental colour of red, and continues to be visible till the impression made upon the retina by the red square has been effaced by other images. By making the same experiment with squares of different colours, it will be found that In these experiments of Buffon, the ground on which the squares of natural colours were viewed, was white, except in the case of the white square, which was placed upon a black ground. Professor Sc herffer has found, that the accidental colours will be much more vivid, and their outlines more distinct, if natural colours are viewed upon a black ground, and the eve transferred to a white gconial. The most convenient way of making the ex periments, is to use coloured wafers, fixed either upon a piece of white or black paper.
In order to explain these phenomena, we must recol lect, that white light is composed of seven different colours, in the following proportions ; the colours being supposed to be arranged in the circumference of a cir cle.
Hence, if we take seven powders of the same colour as the seven prismatic colours, and proportion the quanti ties of each to the numbers in the preceding Table, the mixture of all these powders will be of a white colour ; but if the red powder, or any of the others, be with drawn, the mixture of the remaining colours will not be white, as before. To illustrate this in a more simple
manner, let us suppose, that a circular wheel has its cir cumference divided into seem, s, whose arches are hi the same proportion as the preceding numbers, and that each sector is painted of its proper colour, viz. the sector of SO^ violet, that of 40' indigo, and so on with the rest, as is represented in Plate IV. Fig. 4; then if this wheel be whirled briskly mound its axis, its colour will be white But if the red sector is taken out, or painted black, and the wheel again put in motion, the colour of the wheel will then be green; and, by leaving out the other colours successively, the following results will be obtained: As this experimental method of determining the co lour which arises from mixing any number of the pris matic colours is too circuitous to be used in practice, we shall proceed to point out a method by which the re sulting colour may be determined by a very simple cal culation.
Let the seven prismatic colours he arranged in a cir cle, as in Fig. 4, where each colour occupies its pro per arch of the. circumference; and let us suppose each colour concentrated in the centre of gravity of its arch; then, if we omit any of the colours, it has been found, that the colour resulting from the mixture of all the remaining colours, is that which is nearest to the centre of gravity of the remaining arch. Thus, if we omit violet, the remaining arch will he AEB, whose centre of gravity is the point in, which falls in the green arch : but as the point m does not coincide with g, the centre of gravity of the green arch, the colour arising from a mixture of all the colours, except violet, will not be exactly green, but green mixed with a little yellow, as the point lies between the centres of gravity of the green and yellow arches. Since v is in the centre of the arch AB, and in the centre of AEB, it is evident, that the point ni will always be directly opposite to the centre of gravity v of the violet, or omitted colour ; hence we have only to draw a diameter from the centre of gravity of the omitted colour, and the extremity of that diameter will point out the colour which results from the combination of the rest.