parts, each of which has 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 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 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, 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

0

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.

Charles. And was that the cause of the colours which we saw on some soap bubbles which James was making with a tobacco-pipe? Tutor. It was.

CONVERSATION IX.

Of Colours.

Charles. After what you said yesterday, I am at a loss to know the cause of different colours; the cloth on this table is green; that of which

my coat is made is blue, what makes the difference in these? Am I to believe the poet, that

Colours are but phantoms of the day,

With that they're born, with that they fade away;
Like beauty's charms, they but amuse the sight,
Dark in themselves, till by reflection bright;
With the sun's aid, to rival him they boast,
But light withdraw, in their own shades are lost.

HUGHES.

Tutor. All colours are supposed to exist only in the light of luminous bodies, such as the sun, a candle, &c. and that light falling incessantly upon different bodies is separated into its seven primitive colours, some of which are absorbed, while others were reflected.

James. Is it from the reflected rays that we judge of the colour of objects?

Tutor. It has generally been thought so; thus the cloth on the table absorbs all the rays but the green, which it reflects to the eye; but your coat is of a different texture, and absorbs all but the blue rays.

Charles. Why is paper and the snow white? Tutor. The whiteness of paper is occasioned by its reflecting the greatest part of all the rays that fall upon it. And every flake of snow being an assemblage of frozen globules of water sticking together, reflects and refracts the light that falls upon it in all directions so as to mix it very intimately, and produce a white image on the eye.

James. Does the whiteness of the sun's light arise from a mixture of all the primary colours?

Tutor. It does, as may be easily proved by an experiment, for if any of the seven colours be intercepted at the lens, the image in a great measure loses its whiteness. With the prism I will divide the ray into its seven colours,* I will then take a convex lens in order to re-unite them into a single ray, which will exhibit a round image of a shining white; but if only five or six of these rays be taken with the lens, it will produce a dusky white.

Charles. And yet to this white colour of the sun we are indebted for all the fine colours exhibited in nature:

Fairest of beings! first created light!

Prime cause of beauty! for from thee alone,

The sparkling gem, the vegetable race,

The nobler worlds that live and breathe, their charms, The lovely hues peculiar to each tribe,

From thy unfailing source of splendour draw.

MALLET.

Tutor. These are very appropriate lines, for without light the diamond would lose all its beauty.

James. The diamond, I know, owes its brilliancy to the power of reflecting almost all the

* A figure will be given on this subject with explanations, Conversation XVIII. on the Rainbow.