for I thought the point of a 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.

Tutor. I will explain how these deceptions are managed: let EF(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.

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 contri vances 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 re

commended, 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 Ir. 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 a 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.

James. I once saw a bullock's eye dissected, and was told that it imitated a human eye in its several parts.

Tutor. The eye, when taken from the socket, is of a globular form, and it is composed of three

coats or skins, and three other substances called humours. This figure (Plate 111. Fig. 25.). represents the section of an eye, that is, an eye cut down the middle; and Fig. 26, the front view of an eye as it appears in the head.

Charles. Have these coats and humours all different names?

Tutor. Yes: the external coat, which is represented by the outer circle A B C D E, is called the sclerotica; the front part of this, namely, cx D, is perfectly transparent, and is called the cornea; beyond this, towards B and E, it is white, and called the white of the eye. The next coat, which is represented by the second circle, is called the choroides.

James. This circle does not go all round. Tutor. No: the vacant space a b is that which we call the pupil, and through this alone the light is allowed to enter the eye.

Charles. What do you call that part, which is of a beautiful blue in some persons, as in cousin Lydia; and in others brown, or almost black?

Tutor. That, as a c, be, is part of the choroides, and is called the iris.

Charles. The iris is sometimes much larger than it is at another.

Tutor. It is composed of a sort of net-work, which contracts, or expands according to the force of the light in which it is placed. Let

James stand in a dark corner for two or three minutes:-now look at his eyes.

Charles. The iris of each is very small, and the pupil large.

Tutor. Now let him look steadily, rather close to the candle.

Charles. The iris is considerably enlarged, and the pupil of the eye is but a small point in comparison of what it was before.

Tutor. Did you never feel uneasy after sitting some time in the dark, when candles were suddenly brought into the room?

James. Yes: I remember last Friday evening we had been sitting half an hour almost in the dark at Mr. W's, and when candles were introduced, every one of the company complained of the pain which the sudden light occasioned.

Tutor. By sitting so long in the dark, the iris was contracted very much, of course the pupil being large, more light was admitted than it could well bear, and therefore till time was allowed for the iris to adjust itself, the uneasiness would be felt.

Charles. What do you call the third coat, which, from the figure, appears to be still less than the choroides?

Tutor. It is called the retina, or net-work, which serves to receive the images of objects produced by the refraction of the different hu