Vision [in Opticks]

the physical cause of vision or sight seems to be that the rays of light, striking on the bottom, of the eye, do there excite certain vibrations in the Tunica Retina; which vibrations being propagated, as far as the brain by the solid fibres of th Optick Nerves, do there cause the sense of Seeing.

For as dense bodies do retain their heat longest, and that in proportion to their density, they retain it longer as they are more dense; so the vibrations of their particles are of more durable nature than those of rarer bodies, and can be propagated to greater distances; wherefore the solid and dense fibres of the nerves, whose matter is of an homogeneal and uniform nature, are very proper to transmit to the brain such motions as are impress’d on the external organs of our senses.

For that motion, which can preserve itself a good while in one and the same part of any body, can also be propagated a great way from one part of it to another; provided the body be of an homogeneal nature, and that the motion be not reflected, refracted, interrupted, or disturbed by any inequality in that body.