the wooden ball to the whole globe of the eye; the whole in the ball represents the pupil, the convex lens corresponds to the crystaline humour, and the screen to the retina.

James. The ball by turning in all direc tions is very like the eye, for without moving the head I can look on all sides, and upwards and downwards.

Tutor. Well, we will now place the screen properly, and turn the ball to the garden :-Here you see all the objects perfectly expressed.

James. But they are all inverted.

Tutor. That is the great defect belonging to this instrument; but I will tell you how it may be remedied: take a lookingglass and hold it before you with its face towards the picture on the screen, and inclining a little downwards, and the images will appear erect in the glass, and even brighter than they were on the screen.

• These terms will be explained hereafter.

Gharles. You have shown us in what manner the rays of light are refracted by convex lenses, when those rays are parallel. Will there not be a difference if the rays converge, or diverge before they enter the lens?

Tutor. Certainly if rays converge before they enter a convex lens, they will be collected at a point nearer to the lens than the focus of parallel rays. But if they diverge before they enter the lens, they will then be collected in a point beyond the focus of parallel rays.

There are concave lenses as well as convex, and the refraction which takes place by means of these differs from that which I have already explained.

Charles. What will the effect of refrac tion be, when parallel rays fall upon a double concave lens?

Tutor. Suppose the parallel rays a, b, c, d, &c. (Plate 11. Fig. 14.) pass through the lens A B, they will diverge after they have passed through the glass.

James. Is there any rule for ascertaining the degree of divergency?

OF CONCAVE LENSES.

65

Tutor. Yes; it will be precisely so much as if the rays had come from a radiant point x, which is the centre of the concavity of the glass.

Charles. Is that point called the focus ?

Tutor. It is called the virtual or imaginary focus. Thus the ray a, after passing through the glass A B, will go on in the direction gh, as if it had come from the point x, and no glass been in the way: the ray b, would go on in the direction m n, and the ray e in the direction r s, and so on. The ray CX in the centre suffers no refraction, but proceeds precisely as if no glass had been in the way.

James. Suppose the lens had been concave only on one side, and the other side had been flat, how would the rays have diverged?

Tutor. They would have diverged after passing through it, as if they had come from a radiant point at the distance of a whole diameter of the convexity of the lens. Charles. There is then a great similarit

F 2

in the refraction of the convex and concave

lens.

Tutor. True: the focus of a double convex is at the distance of the radius of convexity, and so is the imaginary focus of the double concave; and the focus of the planoconvex is at the distance of the diameter of the convexity, and so is the imaginary focus of the plano concave.

You will find that images formed by a concave lens, or those formed by a convex lens, where the object is within its principal focus, are in the same position with the objects they represent: they are also imaginary, for the refracted rays never meet at the foci whence they seem to diverge.

But the images of objects placed beyond the focus of a convex lens are inverted, and real, for the refracted rays do meet at their proper foci.

CONVERSATION VIII.

Of the Nature and Advantages of Light------Of the Separation of the Rays of Light by means of a Prism And of compounded Rays, &c.

TUTOR. We cannot contemplate the nature of light without being struck with the great advantages which we enjoy from it. Without that blessing our condition would be truly deplorable.

Charles. I well remember how feelingly Milton describes his situation after he lost his sight:

With the year

Seasons return; but not to me returns