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CONVERSATION XII.

Of Concave Mirrors, and Experi

ments on them.

TUTOR. If you understand what we conversed on yesterday, and what you have yourselves done, you will easily see how the image is formed by the large concave mirror of the reflecting telescope, when we come to examine the construction of that instrument. In a' concave mirror the image is less than the object, when the object is more remote from the

mirror than c, the centre of concavity, and in that case the image is between the object and mirror.

James. Suppose the object be placed in the centre c?

Tutor. Then the image and object will coincide: and if the object is placed nearer to the glass than the centre C, then the image will be more remote, and bigger than the object.

Charles. I should like to see this illustrated by an experiment.

Tutor. Well, here is a large concave mirror: place yourself before it, beyond the centre of the concavity; and with a little care in adjusting your position, you will see an inverted image of yourself in the air between you and the mirror, and of a less size than you are.

When you see the image, extend your hand gently towards the glass, and the hand of . the image will advance to meet it till they both meet in the centre of the glass's concavity. If you carry your hand still farther, the hand of the image will pass by it, and come between it and the body: now move your hand to either side, and the image of it will move towards the other.

James. Is there any rule for finding the distance at which the image of an object is formed from the mirror?

Tutor. If you know the radius of the mirror's concavity, and also the distance of the object from the glass,

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“ Multiply the distance and radius together, and divide the product by double the distance less by the radius, and the quotient is the distance required.”

Tell me at what distance the image of an object will be, suppose the radius of the concavity of the inirror be 12 inches, and the object be at 18 inches froin it.

James. I multiply 18 by 12, which is equal 216; this I divide by double 18 or 36 less by 12, that is 24; but 216 divided by 24 gives 9, which is the number of inches required.

Tutor. You may vary. this example in order to impress the rule on your memory; and I will show you another experiment. I take this bottle, partly full of water, and corked, and place it opposite the concave

mirror, and beyond the focus, that it may appear to be reversed: now stand a little farther distant than the bottle, and you will see the bottle inverted in the air, and the water which is in the lower part of the bottle will appear to be in the upper. I will invert the bottle, and uncork it, and whilst the water is running out, the image will appear to be filling, but when the bottle is empty, the illusion is at an end.

Charles. Are concave mirrors ever used as burning-glasses ?

Tutor. Since it is the property of these mirrors to causé parallel rays to converge to a focus, and since the rays of the sun are considered as parallel, they are very useful as burning-glasses, and the principal focus is the burning point.

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