CONVERSATION I.

INTRODUCTION.

of Light-The Smallness of its Particles-Their Velocity They move only in straight Lines.

CHARLES. When we were on the sea, you told us that you would explain the reason why the oar, which was straight when it lay in the boat, appeared crooked as soon as it was put into the water.

Tutor. I did: but it requires some previous knowledge before you can comprehend the subject. It would afford you but little satisfaction to be told that this deception was caused by the different degrees of refraction which takes place in water and in air.

James. We do not know what you mean by the word refraction.

Tutor. It will therefore be right to proceed with caution; refraction is a term frequently used in the science of optics, and this science depends wholly on light.

James. What is light?

Tutor. It would, perhaps, be difficult to give a direct answer to your question, because we know nothing of the nature of light, but by the effects which it produces. In reasoning, however, on this subject, it is generally admitted that light consists of inconceivably small particles; which are projected, or thrown off from a luminous body with great velocity, in all directions.

Charles. But how is it known that light is composed of small particles?

Tutor. There is no proof indeed that light is material, or composed of particles of matter, and therefore I said it was generally, not universally, admitted to be so; but if it is allowed that light is matter, then the particles must be small beyond all computation, or in falling on the eye they would infallibly blind us.

James. Does not the light come from the sun, in some such manner as it does from a candle?

Tutor. This comparison will answer our purpose; but there appears to be a great difference between the two bodies: a candle, whether of wax or tallow, is soon exhausted; but philosophers have never been able to observe that the body of the sun is diminished by the light which it incessantly pours forth.

James. You say incessantly; but we see only during the hours of day.

Charles. That is because the part of the earth which we inhabit is turned away from the sun

during the night: but our midnight is mid-day to some other parts of the earth.

Tutor. Right: besides you know the sun is not intended merely for the benefit of this globe, but it is the source of light and heat to six other planets, and eighteen moons belonging to them.

Charles. And you have not reckoned the four newly discovered little planets, which Doctor Herschel denominates Asteroids, but which are known by the name of Ceres Ferdinandea, Pallas, Juno, and Vesta.

Tutor. Well then: the sun to these is the perpetual source of light, heat, and motion; and to more distant worlds it is a fixed star, and will appear to some as large as Arcturus, to others no larger than a star of the sixth magnitude, and to others it must be invisible, unless the inhabitants have the assistance of glasses, or are endowed with better eyes than ourselves.

James. Pray, sir, how swift do you reckon that the particles of light move?

Tutor. This you will easily calculate, when you know, that they are only about eight minutes in coming from the sun.

Charles. And if you reckon the sun to be at the distance of ninety-five millions of miles from the earth; light proceeds at the rate, nearly, of twelve millions of miles in a minute, or at 200,000 miles in a second of time. But how do you know that it travels so fast?

Tutor. It was discovered by M. Roemer, who

observed that the eclipses of Jupiter's satellites took place about sixteen minutes later, if the earth were in that part of its orbit, which is farthest from Jupiter, than if it were in the opposite point of the heavens.

Charles. I understand this: the earth may sometimes be in a line between the sun and Jupiter, and at other times the sun is between the earth and Jupiter; and therefore, in the latter case, the distance of Jupiter from the earth is greater than in the former, by the whole length of its orbit.

Tutor. In this situation, the eclipse of any of the satellites is, by calculation, sixteen minutes later than it would be, if the earth were between Jupiter and the sun; that is, the light flowing from Jupiter's satellites is about sixteen minutes in travelling the length of the earth's orbit, or 190 millions of miles.

James. It would be curious to calculate how much faster light travels than a cannon-ball.

Tutor. Suppose a cannon-ball to travel at the rate of twelve miles a minute; and light to move a million of times faster than that; yet Dr. Akenside conjectures that there may be stars so distant from us that the light proceeding from them has not yet reached the earth:

-Whose unfading light

Has travell❜d the profound six thousand years,
Nor yet arriv'd in sight of mortal things.

Charles. Was it to this author that Dr. Young alludes in these lines?

How distant some of the nocturnal suns!
So distant, says the sage, 'twere not absurd
To doubt, if beams, set out on Nature's birth,
Are yet arriv'd at this so foreign world;
Though nothing half so rapid as their flight.

Tutor. He probably referred to Huygens, an eminent astronomer, who threw out the idea before Akenside was born.

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.

VOL. III.-B