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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, 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, 45 of the parts red, 27 orange, 48 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


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.


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sight, Dark in themselves, till by reflection bright; With the sun's aid to rival him they boast, But light withdrawn, in their own shades are lost,


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 are 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

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