mours of the eye, and painted, as it were, on the surface.

Charles. Are the humours of the eye intended for refracting the rays of light, in the same manner as glass lenses?

Tutor. They are; and they are called the vitreous, the crystalline, and the aqueous humours. The vitreous humour fills up all the space z z, at the back of the eye; it is nearly of the substance of melted glass. The crystalline is represented by df, in the shape of a double convex lens: and the aqueous, or watery humour, fills up all that part of the eye between the crystalline humour, and the cornea cx D. James. What does the part A at the back of the eye represent?

Tutor. It is the optic nerve, which serves to convey to the brain the sensations produced on the retina.

Charles. Does the retina extend to the brain? Tutor. It does: and we shall, when we meet next, endeavour to explain the office of these humours in effecting vision. In the meantime, I would request you to consider again what I have told you of the different parts of the eye; and examine, at the same time, both figures; viz. 25 and 26.

James. We will: but you have said nothing about the uses of the eye-brows and eye-lashes. Tutor. I intended to have reserved this to another opportunity: but I may now say, that the

F

from it to receive the rays after they have diverged from E, because every enlightened point of an object becomes visible only by means of a cone of diverging rays from it, and we cease to see it if the rays become parallel or converging. Charles. Is the image inverted?

Tutor. Certainly, because the rays have crossed before they reach the eye.

You may see this subject in another point of view: let ry (Plate 111. Fig. 21.) be a concave mirror, and o the centre of concavity: divide OA equally in F, and take the half, the third, and the fourth, &c. of F o, and mark these divisions,,, &c. Let A o be extended, and parts be taken in it equal to F o, at 2, 3, 4, &c. Now if any of the points 1, 2, 3, 4, &c. be the focus of incident rays, the correspondent points 1, 2, 3, 4, &c. in o F will be the focus of the reflected rays, and vice versa.

James. Do you mean by that, if incident rays be at 2, or or 4, the reflected rays will be at 4?

2, 3,

Tutor. I do place a candle at 2, and an inverted image will be seen at : now place it at 4, and it will also move back to 4: these images may be taken on paper held in those respective places.

Charles. I see the farther you proceed one way with the candle, the nearer its inverted image comes to the point F.

Tutor. True: and it never gets beyond it, for that is the focus of parallel rays after reflection, or of rays that come from an infinite distance. James. Suppose the candle were at o.

Tutor. Then the object and image will coincide: and as the image of an object between F, and a concave speculum, is on the other side of the speculum, this experiment of the candle and paper cannot be made.

I will now just mention an experiment that we may hereafter make. At one end of an oblong box, about two feet long, and 15 inches wide, is to be placed a concave mirror; near the upper part of the opposite end a hole is made, and about the middle of the box is placed a hollow frame of pasteboard, that confines the view of the mirror. The top of the box next the end in which the hole is made is covered with a glass, but the other balf is darkened. Under the hole are placed in succession different pictures, properly painted, which are thrown into perspective by the mirror, and produce a beautiful appearance.

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 III. Fig. 22.) represent a convex mirror, and Af be half the radius of convexity, and take. A F, E 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 .

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 3, 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.