James. What is to make it va

nish?

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 pro

cess 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 gunpowder in the other focus c, it will presently 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 glasses 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 be deformed,

but if an object, as a picture, be painted deformed, according to certain rules, the image will appear regular. Such figures and reflectors are sold by opticians, and they serve to astonish those who are ignorant of these subjects.

CONVERSATION XV.

Of the different Parts of the Eye.

CHARLES. Will you now describe the nature and construction of the telescope?

Tutor. I think it will be better first to explain the several parts of the eye, and the nature of vision in the simple state, before we treat of those instruments which are designed to assist it.