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 a b (Plate 111. Fig. 22.) represent a convex mirror, and Af be half the radius of convexity, and take A F, F 0, 0 E, &c. each equal a f. If incident rays flow from 2, the reflected rays will appear to come from behind the glass

at.

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, *, &c.

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

tude, 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 f in the former is behind the glass, and in the latter it is before the glass as 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 credu-.

lous.

Charles. I remember going with you to see an exhibition in Bondstreet, which you said depended on a concave mirror; I was desired to look into a glass, I did so, and started back, for I thought the point of a

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

dagger would have been in my face. I looked again, and a death's head snapped at me; and then I saw a most beautiful nosegay, which I wished to grasp, but it vanished in an instant.

1

Tutor. I will explain how these deceptions are managed: let, E F (Plate 111. Fig. 23.) be a concave mirror, 10 or 12 inches in diameter, placed in one room; A B the wainscot that separates the spectator from it; but in this there is a square or circular opening which faces the mirror exactly. A nosegay, for instance, is inverted at c, which must be strongly illuminated by means of an Argand's lamp; but no direct light from the lamp is to fall on the mirror. Now a person standing at G will see an image of the nosegay at D.