It seems probable to me, that the limits of the seven primary colours, viz. the extreme red, the limit of the red and orange, of the orange and yellow, yellow and green, green and blue, blue and indigo, indigo and violet, and the extreme violet, excite vibrations in the optic nerve, whose times are proportional to the times of vibration of a string which sounds the notes in order, according to the key mentioned by Sir Isaac Newton in his Optics, i.e. the notes D, E, F, G, A, B, C, D. This hypothesis affords at least a probable reason for the several very particular breadths of the primary colours, in the prismatic oblong image of the sun, as I shall endeavour to show in its place, Prop. 56.
If the frequency of the vibrations excited by the several sapid and odorous bodies in the nerves of the tongue, and membrana Schneideriana, could be discovered, it is not improbable but this would be a clew to lead us into the inner constitution of natural bodies, since one may reasonably suppose, that each sapid and odorous body excites vibrations of the same frequency as those which take place in it before it is tasted or smelt to.
The solution of the same problem in the several senses might also a little unfold to us the different internal structure of the several nerves, and of the parts of the brain that correspond thereto. For it seems probable to me, that each nerve and region is originally fitted to receive, and, as one may say, sympathize with, such vibrations as are likely to be impressed upon them in the various incidents of life; and not that the auditory nerve could perform the office of the optic, if put into its place, or vice versâ, &c. according to Raw.
Eighthly, It is observed by medical writers, that pain is apt to excite a contraction in the fibres of the neighbouring membranes. Now this is very agreeable to that power which sensory vibrations have, in general, over the red fleshy muscles, for contracting them, in the manner to be described hereafter. For since vibrations of a middle strength, by descending into the red fibres of the muscles, are sufficient to contract them in the ordinary functions and actions of life; it is not unreasonable to expect, that the stronger vibrations, which attend pain, should be sufficient to contract the pale fibres of membranes, though these be in themselves of a less contractile disposition.
It is agreeable to this, that titillation and itching, which lie, as it were, upon the confines of pleasure and pain, are more apt to diffuse themselves over the neighbouring parts than pain. For titillation and itching only agitate the small particles of the membranes, and therefore run along their surfaces, by the successive communication of these agitations; whereas, pain, by contracting the fibres, puts a stop to these agitations, and consequently to its own diffusion over the neighbouring parts.
Ninthly, Extreme and pointed parts, such as the extremity of the nose, the uvula, the epiglottis, the nipples, and the ends of
