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Tutor. It does: the intensity or degree of light decreases as the square of the distance from the luminous body increases.

James. Do you mean, that at the distance of two yards from a candle, we shall have four times less light, than we should have, if we were only one yard from it?

Tutor. I do: and at three yards' distance, nine times less light; and at four yards' distance you will have sixteen times less light than you would were you within a yard of the object.--I have one more thing to tell you: light always moves in straight lines.

James. How is that known?

Tutor. Look through a straight tube 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 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.

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CONVERSATION II.

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

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consists of particles flowing from a luminous 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 exist.

Tutor. The sun is perpetually throwing off particles of light, which travel at the rate of twelve millions of iniles 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 þeen 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

now.

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 wilt excite in us those sensations, by which we say have the smell of musk. In the same way particles of light are powing in

we

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