and the images will appear erect in the glass, and even brighter than they were on the screen,

Charles. 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?

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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 refraction 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?

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 bad 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-con

cave.

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.

F

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
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?" How," says a good writer, "could we provide ourselves with food, and the other necessaries of life? How could we transact the least business? How could we correspond with each other, or be of the least reciprocal service without light, and those admirable organs of the body, which the Omnipotent Creator has adapted to the perception of this inestimable benefit?"

James. But you have told us that the light would be of comparatively small advantage without an atmosphere.

Tutor. The atmosphere not only refracts the rays of the light, so that we enjoy longer days than we should without it, but occasions that twilight, which is so beneficial to our eyes; for without it the appearance and disappearance of the sun would have been instantaneous; and in every twenty-four hours we should have experienced a sudden transition from the brightest sun-shine to the most profound darkness, and from thick darkness to a blaze of light.

Charles. I know how painful that would be,
VOL. III.-E

from having slept in a very dark room, and having suddenly opened the shutters when the sun was shining extremely bright.

Tutor. The atmosphere reflects also the light in every direction, and if there were no atmosphere, the sun would benefit those only who looked towards it, and to those whose backs were turned to that luminary it would all be darkness. Ought we not therefore gratefully to acknowledge the wisdom and goodness of the Creator, who has adapted these things to the advantage of his creatures; and may we not with Thomson devoutly exclaim:

How then shall I attempt to sing of Him
Who, light himself, in uncreated light
Invested deep, dwells awfully retired
From mortal eye, or angel's purer ken;
Whose single smile has, from the first of time,
Fill'd, overflowing, all yon lamps of heaven,
That beam for ever through the boundless sky :
But, should He hide his face, th' astonish'd sun,
And all the extinguish'd stars would loosening reel
Wide from their spheres, and Chaos come again.-

James. I saw in some of your experiments that the rays of light, after passing through the glass, were tinged with different colours, what is the reason of this?

Tutor. Formerly light was supposed to be a simple and uncompounded body; Sir Isaac Newton, however, discovered that it was not a simple substance, but was composed of several