quicker of the narrower, overrule each other mutually, so as to compose one tone. But when the vibrations of the extreme rays are greatly different from each other, it seems that each ought to keep the power of exciting its proper vibrations, so as to make the colour of the middle rays; which may be considered as a kind of centre of gravity, a dilute one, verging to white. And white itself, when in perfection, arises from a due proportion of all the sorts of rays, each primary colour, perhaps, keeping its own peculiar vibrations, and the several shades of each primary colour vibrating in the same time as the middle point. When two colours considerably different, as red and blue, yellow and violet, red and violet, are compounded, they neither resemble the intermediate homogeneal one, nor make a white. Not the first, because they are at so great a distance, that each can keep its own vibrations, contrary to what happens in colours resembling homogeneal ones; not a white, because there is not a sufficient number of differing vibrations. By such compositions it is, that purples, and other colours, different from all the homogeneal ones, are formed; and whoever considers the several shades of each colour, with the mutual proportions which may be combined in any compound, may easily conceive how all the colours of natural bodies should arise from mere combinations of the primary colours, agreeably to the sixth and seventh propositions, of the second part of the first book of Sir Isaac Newton’s Optics. What is here delivered may serve to suit the doctrine of vibrations to those propositions, and, perhaps, assist the reader to see the reasons of the sixth.
Cor. If the differences of the primary colours arise from the specific differences of vibrations, it is easy to see, that the differences of tastes and smells may have a like origin; and vice versâ.
Flashes of light, and other luminous appearances, are occasioned by strokes upon the eye, rubbing it, faintings, &c. Now it is very easy to conceive, that violent agitations in the small particles of the optic nerve should arise from these causes; and consequently that such deceptions of the sight, as one may call them, should be produced, if we admit the doctrine of vibrations. And I do not see how they follow from the common hypothesis concerning the manner of sensation.
The most remarkable of these luminous appearances is that which resembles the eye of a peacock’s feather, and which offers itself upon shutting and rubbing the eye in a morning. There is a distinction in it between the central parts and the edges. The first seems to answer to that part of the retina, which is opposite
