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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 ba.

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 D, because that is the line in which the ray last approaches the eye.

James. Is that part of the glass ab 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, 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.

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

medus, at the seige of Syracuse, burned the ships of Marcellus, by a machine composed of mirrors?


Yes: but we have no certain accounts that may be implicitly relied on. Mr. Buffon, about fifty or sixty years ago, burned a plank at the distance of seventy feet, with forty plain mirrors.

James. I do not see how they can act as burning glasses.

Tutor. A plain mirror reflects the light and heat coming from the sun, and will illuminate and heat any substance on which they are thrown, in the same manner as if the sun shone upon it. Two mirrors will reflect on it a double quantity of heat; and if 40 or 100 mirrors could be so placed as to reflect from each the heat coming from the sun, on any particular substance, they would increase the heat 40 or 100 times.


Of Concave Mirrors---their Uses---how they act.

JAMES. To what uses are concave mirrors applied?


They are chiefly used in reflecting telescopes; that is, in tellescopes adapted to viewing the heavenly bodies. And as you like to look at Jupiter's little moons and Saturn's ring through my tellescope, it may be worth your while to take some pains to know by what means this pleasure is afforded you.

Charles. I shall not object to any attention necessary to comprehend the principles on which these instruments are formed.


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