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CONVERSATION X.

Reflected Light, aud Plain Mirrors.

TUTOR. We now come to treat of a different species of glasses, viz. mirrors, of or, as they are sometimes called, specula. James. A looking-glass is a mirror, is it not?

Tutor. Mirrors are made of glass, silvered on one side; they are also made of highly polished metal. There are three kinds of mirrors, the plain, the convex, and the

concave.

Charles. You have shown us that in a booking-glass or plain mirror, " The angle

"of reflection is always equal to the angle "of incident."*

Tutor. This rule is not only applicable to plain mirrors, but to those which are convex and concave also, as I shall show you to-morrow. But I wish to make some observations first on plain mirrors. In the first place, if you wish to see the complete image of yourself in a plain mirror or looking-glass, it must be half as long as you are high.

James. I should have imagined the glass must have been as long as I am high.

Tutor. In looking at your image in the glass, does it not seem to be as far behind the glass as you stand before it.

James. Yes: and if I move forwards or backwards, the image behind the glass seems to approach or recede.

Tutor. Let ab (Plate 11. Fig. 15.) be the looking-glass, and a the spectator, standing opposite to it. The ray from his eye

A

See p. 16.

will be reflected in the same line A a, but the ray cb flowing from his foot, in order to be seen at the eye, must be reflected by the line b A.

Charles. So it will, for if x b be a line perpendicular to the glass, the incident angle will be cbx, equal to the reflected angle A b x.

Tutor. And therefore the foot will appear behind the glass at D along the line A B D, because that is the line in which the ray last approaches the eye.

James. Is that part of the glass a b intercepted by the lines A B and A D, equal ex-' actly to half the lengh of B D, or a c?

Tutor. It is; A ab and A B D may be supposed to form two triangles, the sides of which always bear a fixed proportion to one another; and if A B is double of a a, as, in this case it is, B D will be double of a b, or at least of that part of the glass intercepted by A B and A D.

Charles. This will hold true, I see, stand at what distance we please from the glass. VOL. III. H

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Tutor. If you walk towards a lookingglass, your image will approach, but with a double velocity, because the two motions are equal and contrary. But if, while you stand before a looking-glass, your brother walk up to you from behind, his image will appear to you to move at the same rate as he walks, but to him the velocity of the image will appear to be double; for with regard to you, there will be but one motion, but with regard to him, there will be two equal and contrary ones.

James. If I look at the reflection of a candle in a looking-glass, I see in fact two images, one much fainter than the other, what is the reason of this?

Tutor. The same may be observed of any object that is strongly illuminated, and the reason of the double image is, that a part of the rays are immediately reflected from the upper surface of the glass which form the faint image, while the greater part of them are reflected from the farther surface, or silvering part, and form the vivid image. To see these two images you must stand a

little sideways, and not directly before the glass.

Charles. What is meant by the expression of "An image being formed behind a reflector ?"

Tutor. It is intended to denote that the reflected rays come to the eye with the same inclination as if the object itself were actually behind the reflector. If you, standing on one side of the room, see the image of your brother, who is on the other side, in the looking-glass, the image seems to be formed behind the glass, that is, the rays come to your eye precisely in the same way as they would if your brother himself stood in that place, without the intervention of a glass.

James. But the image in the glass is not so bright or vivid as the object.

Tutor. A plain mirror is in theory supposed to reflect all the light which falls upon it, but in practice nearly half the light is lost on account of the inaccuracy of the po lish, &c.

Charles. Has it not been said, that Archi

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