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rays of light that fall on it: but are vegetable and animal tribes equally indebted to it?

Tutor. What does the gardener do to make his endive and lettuces white?

Charles. He ties them up.

Tutor. That is, he shuts out the light, and by this means they become blanched. I could produce you a thousand instances to show, not only that the colour, but even the existence of vegetables, depend upon light. Close wooded trees have only leaves on the outside, such is the cedar in the garden. Look up the inside of a yew tree, and you will see that the inner branches are almost, or altogether barren of leaves. Geraniums and other green-house plants turn their flowers to the light; and plants in general, if doomed to darkness, soon sicken and die.

James. There are some flowers, the petals of which are, in different parts, of different colours, how do you account for this?

Tutor. The flower of the hearts-ease is of this kind, and if examined with a good microscope, it will be found that the texture of the blue and yellow parts is very different. The texture of the leaves of the white and red rose is also different. Clouds also which are so various in their colours are undoubtedly more or less dense, as well as being differently placed with regard to the eye of the spectator; but the

whole depend on the light of the sun for their beauty, to which the poet refers :

But see, the flush'd horizon flames intense
With vivid red, in rich profusion stream'd
O'er heaven's pure arch.
Their gayest liveries; these with silvery beams
Fring'd lovely; splendid those in liquid gold:

At once the clouds assume

And speak their sovereign's state. He comes, behold! Fountain of light and colour, warmth and life!

The king of glory!

MALLET.

Charles. Are we to understand that all colours depend on the reflection of the several coloured rays of light?

Tutor. This seems to have been the opinion of Sir Isaac Newton; but he concluded from various experiments on this subject, that every substance in nature, provided it be reduced to a proper degree of thinness, is transparent. Many transparent media reflect one colour, and transmit another: gold-leaf reflects the yellow, but it transmits a sort of green colour by holding it up against a strong light.

Mr. Delaval, a gentleman who a few years since made many experiments to ascertain how colours are produced, undertakes to show that they are exhibited by transmitted light alone, and not by reflected light.

James. I do not see how that can be the case with bodies that are not transparent.

Tutor. He infers from his experiments, which you may hereafter examine for yourselves, that the original fibres of all substances, when cleared of heterogeneous matter, are perfectly white, and that the rays of light are reflected from these white particles through the colouring matter with which they are covered, and that this colouring matter serves to intercept certain rays in their passage through it, while a free passage being left to others, they will exhibit, according to these circumstances, different colours.—The red colour of the shells of lobsters after boiling, he says, is only a superficial covering spread over the white calcareous earth, of which the shells are composed, and may be removed by scraping or filing. Before the application of heat, it is so thick as to appear black, being too thick to admit the passage of light to the shell and back again. The case is the same with feathers, which owe their colours to a thin layer of transparent matter on a white ground.

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Reflected Light, and Plain Mirrors.

Tutor. We now come to treat of a different species of glasses, viz. mirrors, or, as they are sometimes called, specula.

James. A looking-glass is a mirror, is it not? Tutor. Mirrors are made of glass, silvered on one side; they are also made of highly polished metal. There are three kinds of mirrors, the plain, the convex, and the concave.

Charles. You have shown us that in a looking-glass or plain mirror, "The angle of reflection is always equal to the angle of incidence."*

Tutor. This rule is not only applicable to plain mirrors, but to those which are convex and concave also, as I shall show you to-morrow. But I wish to make some observations first on plain mirrors. In the first place, if you wish to see the complete image of yourself in a plain mirror or looking-glass, it must be half as long as you are high.

James. I should have imagined the glass must have been as long as I am high.

* See p. 18.

Tutor. In looking at your image in the glass does it not seem to be as far behind the glass a you stand before it.

James. Yes: and if I move forwards or back wards, the image behind the glass seems to ap proach or recede.

Tutor. Let a b (Plate 11. Fig. 15.) be the looking-glass, and A the spectator, standing op posite to it. The ray from his eye will be re flected in the same line A a, but the ray c b flowing from his foot, in order to be seen at the eye, must be reflected by the line b A.

Charles. So it will, for if x b be a line perpendicular to the glass, the incident angle will be c b x, equal to the reflected angle a bx.

Tutor. And therefore the foot will appear behind the glass at D along the line a b D, because that is the line in which the ray last approaches the eye.

James. Is that part of the glass a b intercepted by the lines A B and A D, equal exactly to half the length of B D, or A C?

Tutor. It is; A a b and A B D may be supposed to form two triangles, the sides of which always bear a fixed proportion to one another; and if A B is double of A a, as, in this case it is, BD will be double of a b, or at least of that part of the glass intercepted by A B and A D.

Charles. This will hold true, I see, stand at what distance we please from the glass.

Tutor. If you walk towards a looking-glass,

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