the vibrations of the medullary particles, asserted in the foregoing proposition.
First, then, We are to conceive, that when external objects are impressed on the sensory nerves, they excite vibrations in the æther residing in the pores of these nerves, by means of the mutual actions interceding between the objects, nerves, and æther. For there seem to be mutual actions of all the varieties between these three, in all the senses, though of a different nature in different senses. Thus it seems that light affects both the optic nerve and the æther; and also, that the affections of the æther are communicated to the optic nerve, and vice versâ. And the same may be observed of frictions of the skin, tastes, smells, and sounds. The impulse, attraction, or whatever else be the action of the object, affects both the nerves and the æther; these affect each other, and even the object or impression itself, in most or all cases, so as to alter or modify it. And the result of these actions, upon the whole, may be supposed such a compression or increase of density in the æther, as must agitate its particles with vibrations analogous to those which are excited in the air by the discharge of guns, by thunderclaps, or by any other method of causing a sudden and violent compression in it.
Secondly, We are to conceive, that the vibrations thus excited in the æther will agitate the small particles of the medullary substance of the sensory nerves with synchronous vibrations, in the same manner as the vibrations of the air in sounds agitate many regular bodies with corresponding vibrations or tremblings. And here the uniformity, softness, and active powers of the medullary substance, must be considered as previous requisites and assistances. A want of uniformity in the medullary substance would argue a like want of uniformity in the æther contained within it. The hardness of it, if it extended to the particles, would cause an ineptitude to vibratory motions in the particles of these particles, i.e. in the infinitesimal particles considered in this and the foregoing proposition. And a want of active powers in these particles would suffer the excited motions to die away prematurely.
One may conjecture, indeed, that the rays of light excite vibrations in the small particles of the optic nerve, by a direct and immediate action. For it seems probable, from the alternate fits of easy transmission and reflexion, that the rays of light are themselves agitated by very subtle vibrations, and consequently that they must communicate these directly and immediately to the particles of the optic nerves. And it may be also, that sapid and odoriferous particles are agitated with specific vibrations, and that they communicate these directly and immediately to the small particles of the gustatory and olfactory nerves respectively, as well as to the interjacent æther. Upon this supposition, the vibrations of the æther must be conceived as regulating
