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

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 raya, after passing through the glass A B, will go on in the direction g h, 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 c x 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 similarity 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 plano-convex 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 sarme 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.


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 Day, or the sweet approach of ev'n or morn, Or sight of vernal bloom, or summer's rose, Or flocks, or herds, or human face divine; But cloud instead, and ever-during dark Surrounds me, from the cheerful ways of men Cut off, and for the book of knowledge fair, Presented with an universal blank Of Nature's works, to me expung’d and raz’d, And wisdom, at one entrance, quite shut out.

Tutor. Yet his situation was rendered comfortable by means of friends and relations, who all possessed the advantages of light. But if our world were deprived of light, what pleasure or even comfort could we enjoy. 1" How,” says a good writer, “ could we provide ourselves with food, and the other necessaries of life? How could we transact the least busiiess? How could we correspond

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