by the nerves, impart sense and motion to all parts of the body.
The instruments of motion are the muscles; the fibres whereof contracting themselves, move the several parts of the body.
This contraction of the muscles is, in some of them, by the direction of the mind, and in some of them without it; which is the difference between voluntary and involuntary motions, in the body.
CHAPTER XI.
Of the Five Senses.
OF SEEING.
The organ of seeing is the eye; consisting of variety of parts wonderfully contrived, for the admitting and refracting the rays of light; so that those that come from the same point of the object, and fall upon different parts of the pupil, are brought to meet again at the bottom of the eye, whereby the whole object is painted on the retina that is spread there.
That which immediately affects the sight, and produces in us that sensation which we call seeing, is light.
Light may be considered either, first, as it radiates from luminous bodies directly to our eyes; and thus we see luminous bodies themselves, as the sun, or a flame, &c. or secondly, as it is reflected from other bodies; and thus we see a man, or a picture, by the rays of light reflected from them to our eyes.
Bodies, in respect of light, may be divided into three sorts; first, those that emit rays of light, as the sun and fixed stars; secondly, those that transmit the rays of light, as the air; thirdly, those that reflect the rays
