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 fixt stars; secondly, those that transmit the rays of light, as the air; thirdly, those that reflect the rays of light, as iron, earth, &c. The first are called luminous; the second pellucid; and the third opake.

The rays of light themselves are not seen; but by them the bodies, from which they originally come; as the sun, or a fixt star; or the bodies, from which they are reflected; as a horse, or a tulip. When the moon shines, we do not see the rays which come from the sun to the moon, but by them we see the moon, from whence they are reflected.

If the eye be placed in the medium, through which the rays pass to it, the medium is not seen at all; for instance, we do not see the air through which the rays come to our eyes. But if a pellucid body, through which the light comes, be at a distance from our eye, we see that body, as well as the bodies, from whence the rays come that pass through them to come to our eyes. For instance, we do not only see bodies through a pair of spectacles, but we see the glass itself. The reason whereof is, that pellucid bodies being bodies, the surfaces of which reflect some rays of light from their solid parts; these surfaces, placed at a convenient distance from the eye, may be seen by those reflected rays; as, at the same time, other bodies beyond those pellucid ones may be seen by, the transmitted rays.

Opake bodies are of two sorts, specular, or not specular. Specular bodies, or mirrours, are such opake bodies, whose surfaces are polished; whereby they, reflect

ing the rays in the same order as they come from other bodies, show us their images.

1

The rays that are reflected from opake bodies, always bring with them to the eye the idea of colour; and this colour is nothing else, in the bodies, but a disposition to reflect to the eye more copiously one sort of rays than another. For particular rays are originally endowed with particular colours; some are red, others blue, others yellow, and others green, &c.

Every ray of light, as it comes from the sun, seems a bundle of all these several sorts of rays; and as some of them are more refrangible than others; that is, are more turned out of their course, in passing from one medium to another; it follows, that after such refraction they will be separated, and their distinct colour observed. Of these, the most refrangible are violet, and the least red; and the intermediate ones, in order, are indigo, blue, green, yellow, and orange. This separation is very entertaining, and will be observed with pleasure in holding a prism in the beams of the sun.

As all these rays differ in refrangibility, so they do in reflexibility; that is, in the property of being more easily reflected from certain bodies, than from others; and hence arise, as hath been said, all the colours of bodies; which are, in a manner, infinite, as an infinite number of compositions and proportions, of the original colours, may be imagined.

The whiteness of the sun's light is compounded of all the original colours, mixed in a due proportion.

Whiteness, in bodies, is but a disposition to reflect all colours of light, nearly in the proportion they are mixed in the original rays; as, on the contrary, blackness is only a disposition to absorb or stifle, without reflection, - most of the rays of every sort that fall on the bodies.

Light is successively propagated with an almost inconceivable swiftness; for it comes from the sun, to this our earth, in about seven or eight minutes of time, which distance is about 80,000,000 English miles.

Besides colour, we are supposed to see figure, but, in truth, that which we perceive when we see figure, as perceiveable by sight, is nothing but the termination of colour.

OF HEARING.

NEXT to seeing, hearing is the most extensive of our senses. The ear is the organ of hearing, whose curious structure is to be learnt from anatomy.

That which is conveyed into the brain by the ear is 'called sound; though, in truth, till it come to reach and affect the perceptive part, it be nothing but motion.

The motion, which produces in us the perception of sound, is a vibration of the air, caused by an exceeding short, but quick, tremulous motion of the body, from which it is propagated; and therefore we consider and denominate them as bodies sounding.

That sound is the effect of such a short, brisk, vibrat ing motion of bodies, from which it is propagated, may be known from what is observed and felt in the strings of instruments, and the trembling of bells, as long as we perceive any sound come from them; for as soon as that vibration is stopt, or ceases in them, the perception ceases also.

The propagation of sound is very quick, but not approaching that of light. Sound moves about 1140 English feet in a second of time; and in seven or eight minutes of time, they move about one hundred English *miles.

OF SMELLING.

SMELLING is another sense, that seems to be wrought on by bodies at a distance; though that which immediately affects the organ, and produces in us the sensation of any smell, are affluvia, or invisible particles, that coming from bodies at a distance, immediately affect the 'olfactory nerves.

Smelling bodies seem perpetually to send forth effluvia, or steams, without sensibly wasting at all. Thus a grain of musk will send forth odoriferous particles for scores of years together, without its being spent; whereby one would conclude that these particles are very small;

and

and yet it is plain, that they are much grosser than the rays of light, which have a free passage through glass; and grosser also than the magnetic affluvia, which pass freely through all bodies, when those that produce smell will not pass through the thin membranes of a bladder, and many of them scarce ordinary white paper.

There is a great variety of smells, though we have but a few names for them; sweet, stinking, sour, rank, and musty, are almost all the denominations we have for odours; though the smell of a violet, and of musk, both called sweet, are as distinct as any two smells what

soever.

OF TASTE.

TASTE is the next sense to be considered.
The organ of taste is the tongue and palate.

Bodies that emit light, sounds, and smells, are seen, heard, and smelt at a distanee; but bodies are not tasted, but by immediate application to the organ; for till our meat touch our tongues, or palates, we taste it not, how near soever it be.

It may be observed of tastes, that though there be a great variety of them, yet, as in smells, they have only some few general names; as sweet, bitter, sour, harsh, rank, and some few others.

OF TOUCH.

THE fifth and last of our senses is touch; a sense spread over the whole body, though it be most eminently placed in the ends of the fingers.

By this sense the tangible qualities of bodies are discerned; as hard, soft, smooth, rough, dry, wet, clammy, and the like.

But the most considerable of the qualities, that are perceived by this sense, are heat and cold..

The due temperament of those two opposite qualities, is the great instrument of nature, that she makes use of in most, if not all, her productions.

Ff3

Heat

Heat is a very brisk agitation of the insensible parts of the object, which produces in us that sensation, from whence we denominate the object hot; so what in our sensation is heat, in the object is nothing but motion. This appears by the way whereby heat is produced; for we see that the rubbing of a brass nail upon a board will make it very hot, and the axle-tree of carts and coaches are often hot, and sometimes to a degree, that it sets them on fire, by the rubbing of the nave of the wheel upon it.

On the other side, the utmost degree of cold is the cessation of that motion of the insensible particles, which to our touch is heat.

Bodies are denominated hot and cold in proportion to the present temperament of that part of our body to which they are applied; so that feels hot to one, which seems cold to another; nay, the same body, felt by the two hands of the same man, may at the same time appear hot to the one, and cold to the other; because the motion of the insensible particles of it may be more brisk than that of the particles of the other.

Besides the objects before-mentioned, which are peculiar to each of our senses, as light, and colour of the sight; sound of hearing; odours of smelling; savours of tasting; and tangible qualities of the touch; there are two others that are common to all the senses; and those are pleasure and pain, which they may receive by and with their peculiar objects. Thus, too much light offends the eye; some sounds delight, and others grate the ear; heat in a certain degree is very pleasant, which may be augmented to the greatest torment; and so the

rést,

These five senses are common to beasts with men; nay, in some of them, some brutes exceed mankind. But men are endowed with other faculties, which far excel any thing that is to be found in the other animals in this our globe.

Memory also brutes may be supposed to have, as well

as men.

CHAP