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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.
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.
Charles. I do not quite comprehend in what manner reflection takes place at a convex mirror.
Tutor. I will endeavour by a figure to make it plain: C D (Plate III. Fig. 19.) represents a convex mirror standing at the end of a room, before which the arrow A B is placed on one side or obliquely: where must the spectator stand to see the reflected image?
Charles. On the other side of the room.
Tutor. The eye E will represent that situation: the rays from the external parts of the arrow, A and B, flow convergingly along ▲ a and в b, and if no glass were in the way they would meet at P; but the glass reflects the ray A a along a E, and the ray в b along
E; and as we always transfer the image of an object in that direction in which the rays approach the eye, we see the image of A along the line E a behind the glass, and the image of B along E b, and, of course, the image of the whole arrow is at s.
By means of a similar diagram, I will show you more clearly the principle of the concave mirror. Suppose an object e (Plate 111. Fig. 20.) to be beyond the focus, F, and the spectator to stand at z, the rays e b and e d are reflected, and where they meet in å the spectator will see the image.
James. That is between himself and the object.
Tutor. He must, however, be far enough from it to receive the rays after they have diverged from E, because every enlightened point of an object becomes visible only by means of a cone of diverging rays from it, and we cease to see it if the rays become parallel or converging.
Charles. Is the image inverted?
Tutor. Certainly, because the rays have crossed before they reach the eye.
You may see this subject in another point of view: let ry (Plate 111. Fig. 21.) be a concave mirror, and o the centre of concavity divide o A equally in F, and take the half, the third, and the fourth, &c. of r o,