« PreviousContinue »
Tutorl Look through a straight tube
Tutor at any object, and the rays of light will flow readily from it to the eye, but let the tube
be bent, and the object cannot be seen ị through it, which proves that light will move only in a straight line.
This is plain also from the shadows which opaque bodies cast; for if the light did not describe straight lines, there would be no shadow. Hold any object in the light of the sun, or a candle, as a square board or book, and the shadow caused by it proves that light moves only in right or straight lines.
Of Rays of Light. Of Reflection and Refraction.
CHARLES. You talked, the last time we met, of the rays of light flowing or moving, what do you mean by a ray of light?
Tutor. Light you know is supposed to be made up of indefinitely small particles ; now one or more of these particles in motion from any body, is called a ray of light.
-If the supposition be true, that light consists of particles flowing from a lumi. nous body, as the sun, and that these particles are about eight minutes in coming
from the sun to us; then if the sun were blotted from the heavens, we should actually have the same appearance for eight minutes after the destruction of that body as we now have.
James. I do not understand how we could see a thing that would not ist.
Tutor. The sun is perpetually throwing off particles of light, which travel at the rate of twelve millions of miles in a minute, and it is by these that the image of the body is impressed on our eye. The sun being blotted from the firmament would not affect the course of the particles that had the instant before been thrown from his body, they would travel on as if nothing had happened, and till the last
particles had reached the eye, we should think we saw the sun, as much as we do
Charles. Do we not actually see the body itself?
Tutor. The sense of sight may, perhaps, not be unaptly compared to that of smell : a grain of musk will throw off its odoriferous particles all round, to a considerable distance, now if you or I happen to be near it, the particles which fall upon certain nerves in the nose will excite in us those sensations, by which we say we have the smell of musk. In the same way particles of light are flowing in every direction from the grain of musk, some of which fall on the eye, and these excite different sensations, from which we say, we see a piece of musk.
Charles. But the musk will in time be completely dissipated, by the act of throwing off the fine particles ; whereas a chair table may
throw off its rays so as to be visible, without ever diminishing its size.
Tutor. True : because whatever is distinguished by the sense of smell, is known only by the particles of the odoriferous body itself flowing from it: whereas a body distinguished by the sense of sight is known by the rays of light, which first
fall on the body, and are then reflected from it.
James. What do you mean by being reflected ?
Tutor. If I throw this marble smartly against the wainscot, will it remain wherë it was thrown?
James. No : it will rebound, or come back again.
Tutor. What you call rebounding, writers on optics denominate reflection. When a body of any kind, whether it be a marble with which you play, or a particle of light, strikes against á surface, and is sent back again, it is said to be reflected. If you shoot a marble straight against a board, or other obstacle, it comes back in the same line, or nearly so ; but suppose you throw it sideways, does it return to the hand ?
Charles. Let me see: I will shoot this marble against the middle of one side of the room, from the corner of the opposite side.