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2, the reflected rays will appear to come

from behind the glass at

James. Do you mean if a candle be pla

ced at 2, the image of it will appear to be formed at behind the glass? 148

Tutor. I do and if that, or any other object, be carried to 3, 4, &c. the image will also go backward to 1, 1, &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 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 credulous.

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 dagger would have been in my face. I look

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

VOL. III.

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ed again, and a death's head snapped at 12 and then I saw a most beautiful noseg which I wished to grasp, but it vanished an instant.

Tutor. I will explain how these dece tions are managed: let E F (Plate 111. Fi 23.) be a concave mirror, 10 or 12 inch in diameter, placed in one room; A B th wain-scot that separates the spectator from it; but in this there is a square or circula 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.

James. What is to make it vanish?

Tutor. In exhibitions of this kind there is always a person behind the wainscot in league with the man that attends the spectator, who removes the real nosegay upon some hint understood between them.

Charles. Was it then upon the man behind the scene that the approaching sword, and the, advancing death's head, &c. depended?

Tutor. It was: and persons have undertaken to exhibit the ghosts of the dead by contrivances of this kind: for if a drawing of the deceased be placed instead of the nosegay, it may be done. But such exhibitions are not to be recommended, and indeed ought never to be practised, without explaining the whole process to the astonished spectator afterwards.

If a large concave mirror be placed before a blazing fire so as to reflect the image of the fire on the flap of a bright mahogany table, a spectator suddenly introduced in the room will suppose the fire to be on the table.

If two large concave mirrors A and B (Plate 111. Fig. 24.) be placed opposite each other, at the distance of several feet, and red hot charcoal be put in the focus D, and some gun-powder in the other focus c, it will pre

sently take fire. The use of a pair of bellows may be necessary to make the charcoal burn strongly.

This experiment may be varied by placing a thermometer in one focus, and lighted charcoal in the other, and it will be seen that the quicksilver in the thermometer will rise as the fire increases, though another thermometer at the same distance from the fire, but not in the focus of the glass, will not be affected by it.

James. I have seen concave glases in which my face has been rendered as long as my arm, or as broad as my body, how are these made?

Tutor. These images are called anamorphoses, and are produced from cylindrical concave mirrors; and as the mirror is placed either upright, or on its side, the image of the picture is distorted into a very long or very broad image.

Reflecting surfaces may be made of various shapes, and if a regular figure be placed before an irregular reflector, the image will

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