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, 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 : f

How then shall I attempt to sing of Him
Who, light himself, in uncreated light
Invested deep, dwells awfully retir'd
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

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 parts, each of which has in fact a different degree of refrangibility.

Charles. How is that shown?

Tutor. Let the room be darkened, and let there only be a very small hole in the shutter to admit the sun's rays; instead of a lens I take a triangular piece of glass, called a prism; now as in this there is nothing to bring the rays to a focus, they will, in passing through it, suffer different degrees of refraction, and be separated into the different coloured rays, which being received on a sheet of white paper will exhibit the seven following colours: red, orange, yellow, green, blue, indigo, and violet; and now you shall hear a poet's description of them.

First the flaming red

Sprung vivid forth; the tawny orange next;
And next delicious yellow; by whose side
Fell the kind beams of all-refreshing green.
Then the pure blue, that swells autumnal skies.
Ethereal play'd; and then of sadder hue,
Emerg'd the deepen'd indigo, as when
The heavy skirted evening droops with frost,
While the last gleamings of refracted light
Dy'd in the fainting violet away.

THOMSON.

James.

Here are all the colours.

of the rainbow: the image on the

paper is a sort of oblong.

Tutor. That oblong image is usually called a spectrum, and if it be divided into 360 equal parts, the red will occupy 45 of them, the orange 27, the yellow 48, the green and the blue 60 each, the indigo 40, and the violet 80.

VOL. V.