The gay colouring with which the Almighty has decked the pale marble of nature is not the result of any quality inherent in the coloured body, or in the particles by which it may be tinged, but is merely a property of the light in which they happen to be placed. Newton was the first person who placed this great truth in the clearest evidence. He found that all bodies, whatever were their peculiar colours, exhibited these colours only in white light. When they were illuminated by homogeneous _red_ light they appeared _red_, by homogeneous _yellow_ light, _yellow_, and so on, “their colours being most brisk and vivid under the influence of their own daylight colours.” The leaf of a plant, for example, appeared _green_ in the white light of day, because it had the property of reflecting that light in greater abundance than any other. When it was placed in homogeneous _red_ light, it could no longer appear _green_, because there was no green light to reflect; but it reflected a portion of red light, because there was some red in the compound green which it had the property of reflecting. Had the leaf originally reflected a pure homogeneous green, unmixed with red, and reflected no white light from its outer surface, it would have appeared quite black in pure homogeneous red light, as this light does not contain a single ray which the leaf was capable of reflecting. Hence the colours of material bodies are owing to the property which they possess of stopping certain rays of white light, while they reflect or transmit to the eye the rest of the rays of which white light is composed. So far the Newtonian doctrine of colours is capable of rigid demonstration; but its author was not content with carrying it thus far: he sought to determine the manner in which particular rays are stopped, while others are reflected or transmitted; and the result of this profound inquiry was his theory of the colours of natural bodies, which was communicated to the Royal Society on the 10th February, 1675. This theory is perhaps the loftiest of all his speculations; and though, as a physical generalization, it stands on a perishable basis, and must soon be swept away in the progress of science, it yet bears the deepest impress of the grasp of his powerful intellect. The principles upon which this theory is founded are the following:— 1. Bodies that have the greatest refractive powers reflect the greatest quantity of light; and at the confines of equally refracting media there is no reflection. 2. The least particles of almost all natural bodies are in some measure transparent. 3. Between the particles of bodies are many pores or spaces, either empty or filled with media of less density than the particles. 4. The particles of bodies and their pores, or the spaces between the particles, have some definite size. Upon these principles Newton explains the origin of _transparency_, _opacity_, and _colour_.