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those only who looked towards it, and to those whose backs were turned to that lu. minary 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 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 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 .hrough 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 hibit the seven following colours : red, orange, yellow, green, blue, indigo, and violet ; and now you shall hear a poet's des cription of them.

will ex

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

Fames. 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 forty-five of them, the orange twenty-seven, the yellow forty-eight, the green and the blue sixty each, the indigo forty, and the violet eighty.

Charles. The shade of difference in some of these colours seems very small indeed.

Tutor. You are not the only person who has made this observation; some experimental philosophers say there are but three original and truly distinct colours, viz, the red, yellow, and blue.

Charles. What is called the orange is surely only a mixture of the red and yellow, between which it is situated.

Tutor. In like manner the green is said to be a mixture of the yellow and blue, and the violet is but a fainter tinge of the indigo.

James. How is it then that light which consists of different colours, is usually seen as white?

Tutor. By mixing the several colours in due proportion white may be produced. James. Do you mean to say

that a mixture of red, orange, yellow, green, blue, VOL. 113.

G

indigo, and violet, in any proportion, will produce a white ? Tutor. If

you

divide a circular surface into 360 parts, and then paint it in the proportion just mentioned, that is, forty-five of the parts red, twenty-seven orange, forty-eight yellow, &c. and turn it round with great velocity, the whole will appear of a dirty white, and if the colours were more perfect the white would be so too.

James. Was it then owing to the separation of the different rays, that I saw the rainbow colours about the edges of the image made with the lens ?

Tutor. It was: some of the rays were scattered, and not brought to a focus, and these were divided in the course of refraction. And I may tell you now, though I shall not explain it at present, that the rainbow in the heavens is caused by the separation of the rays of light into their component parts.

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