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James. There are some flowers, the pe tals of which are, in different parts, of dif ferent 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 depends on the light of the sun for their beauty, to which the poet refers :
But see, the flush'd horizon flames intense
Charles. Are we to understand that all colours depend on the reflection of the seve ral 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 transpa
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 parti
cles 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 or transparent matter on a white ground.
TUTOR. We now come to treat of a different species of glasses, viz. mirrors, of 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
Charles. You have shown us that in a booking-glass or plain mirror, “The angle
"of reflection is always equal to the angle "of incident."*
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.
Tutor. In looking at your image in the glass, does it not seem to be as far behind the glass as you stand before it.
James. Yes and if I move forwards or backwards, the image behind the glass seems to approach or recede.
Tutor. Let ab (Plate 11. Fig. 15.) be. the looking-glass, and a the spectator, standing opposite to it. The ray from his eye
See p. 16.