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Fames. You see, instead of coming back to your hand, it goes off to the other corner, directly opposite to the place from which
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.
Tytor. 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
Charles. I see there are two angles, and hey 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 Charles. Then if I go to that part, I shall see the rays of light flowing from my brother :
-and I do see him in the glass.
James. And I see Charles.
Tutor. Now the rays of light flow from each of you to the glass, and are reflected to one another : but neither of you sees himself.
Charles. No: I will move in front of the glass, now I see myself, but not my brother; and, I think, I understand the subject very well.
Tutor. Then explain it to me by a figure, which you may draw on the slate.
Charles. Let A B (Plate 1. Fig. 1.) represent the looking-glass : if I stand at C, the rays flow from me to the glass, and are reflected back in the same line, because now there is no angle of incidence, and of course no angle of reflection ; but if I stand at x, then the rays flow from me VOL. III.