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 or a 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 where 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 a 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.

James. You see, instead of coming back to your hand, it goes off to the other corner, directly opposite to the place from which you sent it.

Tutor. This will lead us to the explanation of one of the principal definitions in optics, viz. that the angle of reflection is always equal to the angle of incidence. You know what an angle is ?*

Charles. We do but not what an angle of incidence is.

Tutor. I said a ray of light was a particle of light in motion: now there are incident rays, and reflected rays.

The incident rays are those which fall on the surface; and the reflected rays are those which are sent off from it,

Charles. Does the marble going to the wainscot represent the incident ray, and in going from it, does it represent the reflected ray?

* See Scientific Dialogues, Vol. I. Conversation I.

Tutor. It does: and the wainscot may be called the reflecting surface.

James. Then what are the angles of incidence and reflection?

Tutor. Suppose you draw the lines on which the marble travelled, both to the wainscot, and from it again.

Charles. I will do it with a piece of

chalk.

Tutor.

Now draw a perpendicular* from the point where the marble struck the surface, that is, where your two lines

meet.

Charles. I see there are two angles, and they seem to be equal.

Tutor. We cannot expect mathematical precision in such trials as these; but if

* If the point be exactly in the middle of one side of the room, a perpendicular is readily drawn by finding the middle of the opposite side, and joining the two points.

the experiment were accurately made, the two angles would be perfectly equal: the angle contained between the incident ray, and the perpendicular, is called the angle of incidence, and that contained between the perpendicular and reflected ray, is called the angle of reflection.

James. Are these in all cases equal, shoot the marble as you will?

Tutor. They are: and the truth holds equally with the rays of light :-both of you stand in front of the looking-glass. You see yourselves, and one another also; for the rays of light flow from you to the glass, and are reflected back again in the same lines. Now both of you stand on one side of the room. What do you see?

Charles. Not ourselves, but the furniture on the opposite side.

Tutor. The reason of this is, that the rays of light flowing from you to the glass, are reflected to the other side of the

room.