James. And you say the particles of light move in all directions.

Tutor. Here is a sheet of thick brown paper, I make only a small pin-hole in it, and then through that hole, I can see all the objects, such as the sky, trees, houses, &c. as I could if the paper were not there.

Charles. Do we only see objects by means of the rays of light which flow from

them?

Tutor. In no other way: and therefore the light that comes from the landscape, which I view by looking through the small hole in the paper, must come in all directions at the same time.-Take another instance; if a candle be placed on an eminence in a dark night, it may be seen all round for the space of half a mile in other words, there is no place within a sphere of a mile in diameter, where the candle cannot be seen, that is, where some of the rays from the small flame will not be found.

:

James. Why do you limit the distance to half a mile?

Tutor. The distance, of course, will be greater or less, according to the size of the candle: but the degree of light, like heat, diminishes in proportion as you go farther from the luminous body.

Charles. Does it follow the same law as gravity?*

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?

• See Scientific Dialogues, Vol. I. Conversation

Tutor 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 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.

B 2

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 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 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

now.

Charles.

body itself?

Do we not actually see the

Tutor. The sense of sight may, perhaps, not be unaptly compared to that of