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 drawing-rooms: 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 xx.

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 mir

ror.

Tutor. I will endeavour, by a figure, to make it plain: CD (Plate II. 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 Aa and B 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 Bb along bE; 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 n. Fig. 20.) to be beyond the focus F, and the spectator to stand at z, the rays e b and ed are reflected, and where they meet in E the spectator will see the image.

James. That is between himself and the object.

Tutor. He must, however, be far enough VOL. III.-G

CONVERSATION XIV.

Of Convex Reflection-Of Optical Delusions-Of Anamorphoses.

Charles. You cannot, I see, make the same experiment with the candle, and a convex mirror, that you made yesterday with the concave

one.

Tutor. Certainly, because the image is formed behind the glass: but it may, perhaps, be worth our while to consider how the effect is produced in a mirror of this kind. Let ab (Plate 111. Fig. 22.) represent a convex mirror, and Af be half the radius of convexity, and take. A F, F 0, 0 B, &c. each equal a f. If incident rays flow from 2, the reflected rays will appear to come from behind the glass at 4.

James. Do you mean if a candle be placed at 2, the image of it will appear to be formed at behind the glass ?

Tutor. I do and if that, or any other object, be carried to 3, 4, &c. the image will also go backward to, 4, &c.

Charles. Then, as a person walks towards a convex spherical reflector, the image appears to walk towards him, constantly increasing in

magnitude, till they touch each other at the surface.

Tutor. You will observe that the image, however distant the object, is never farther off than at f; that is, the imaginary focus of parallel

rays.

James. The difference then between convex and concave reflectors is, that the point ƒ in the former is behind the glass, and in the latter it is before the glass at F.

Tutor. Just so: from the property of diminishing objects, spherical reflectors are not only pleasing ornaments for our best rooms, but are much used by all lovers of picturesque scenery. "Small convex reflectors," says Dr. Gregory, "are made for the use of travellers, who, when fatigued by stretching the eye to Alps towering on Alps, can, by their mirror, bring these sublime objects into a narrow compass, and gratify the sight by pictures which the art of man in vain attempts to imitate."*

Concave mirrors have been used for many other and different purposes; for by them, with a little ingenuity, a thousand illusions may be practised on the ignorant and credulous.

Charles. I remember going with you to see an exhibition in Bond street, which you said depended on a concave mirror; I was desired to look into a glass, I did so, and started back,

* See Economy of Nature, Vol. I. p. 26, 2d Edition.