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124. The Sun-light once broken by a prism does not admit of any farther decomposition of the same kind; for if a portion of the coloured light of any sort be made to pass through a second prism it preserves its peculiar colour unchanged, and the beam of light, has, after refraction, the same form as before. For this reason the rays of any one colour are called homogeneous, and the light simple or homogeneous light.
125. It is remarkable that an object illuminated by homogeneous light has no colour but that of the particular sort of light falling on it; the only distinction observable being that an object naturally of the same colour as the light, appears brighter than one of a different hue though much less so than a white one. This leads one to conjecture that the colours of natural substances are owing to some power residing in them whereby they decompose the ordinary compound light, and reflect only some particular kinds.[1] This conjecture is strengthened by an experiment made by Sir Isaac Newton, in which he found that when a piece of paper coloured partly red and partly blue, and marked with black lines, was illuminated by a candle and a convex-lens of considerable focal distance placed before it, the images of the two parts were not adjacent, but the blue was at a greater distance from the lens than the red, just as if the paper had been illuminated partly with red and partly with blue homogeneous light.
126. It will readily be conceived that the unequal refrangibility of light must give rise to a good deal of confusion in all images produced by refraction. For instance, a pencil of rays diverging from a point on the axis of a convex-lens will be collected, not to one single focus, but to a series of foci lying at different distances from the lens along its axis, and there will consequently be a confusion analogous to that arising from the aberration which we found to be produced by the spherical form of a refracting surface.
- ↑ According to this theory we may suppose that a substance appears perfectly white when it reflects all the rays of light indiscriminately, and grey or black when it absorbs some of all, and reflects more or less of the compound light without decomposing or dividing it. Sir Isaac Newton confirmed this by showing that a grey paper placed in sunshine appeared brighter, that is, more white, than a white one placed near it in the shade.
