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,

"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 mirror be 12 inches, and the object be at 18 inches from 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.

Tutor. Since it is the property of these mirrors to cause 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.

James. Is the image formed by a concave mirror always before it?

Tutor. In all cases, except when the object is nearer to the mirror than the principal focus.

Charles. Is the image then behind the mirror?

Tutor. It is; and farther behind the mirror than the object is before it. Let a c (Plate 111. Fig. 18.) be a mirror, and x z the object between the centre K of the glass, and the glass itself; and the image x y z will be behind the glass erect, curved, and magnified, and of course the image is farther behind the glass than the object is before it.

James. What would be the effect if, instead of an opaque object x z, a luminous one, as a candle, were placed in the focus of a concave mirror?

Tutor. It would strongly illuminate a space of the same dimension as the mirror to a great distance: and if the candle were still nearer the mirror than the focus, its rays will enlighten a larger space. Hence you may understand the construction of many of the lamps which are now to be seen in many parts of London, and which are undoubtedly a great improvement in lighting the streets.

CONVERSATION XIII.

Of Concave and Convex Mirrors.

TUTOR. We shall devote another morning or two to the subject of reflection from mirrors of different kinds.

Charles. You have not said any thing about convex mirrors, and yet they are now very much in fashion in handsome drawingrooms: I have seen several, and always observed that the image was very much less than the object.

Tutor. A convex mirror is an ornamental piece of furniture, especially if it can be placed before a window, either with a good

prospect, or where there are a number of persons passing and repassing in their different employments. The images reflected from these are smaller than the objects, erect, and behind the surface, therefore a landscape or a busy scene delineated on one of them, is always a beautiful object to the eye. For the same reason a glass of this kind in a room in which large assemblies meet, forms an extremely interesting picture. You may easily conceive how the convex mirror diminishes objects, or the images of objects, by considering in what manner they are magnified by the concave mirror. If x y z (Fig. 18.) were a straight object before a convex mirror A c, the image by reflection would be x z.

James. Would it not appear curved?

Tutor. Certainly for if the object be a right line, or a plain surface, its image must be curved, because the different points of the object are not equally distant from the reflector. In fact, the images formed by convex mirrors, if accurately compared with the objects, are never exactly of the same shape.