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ing position. You see that the immersed part appears as if it were bent upwards, towards the surface; but I press it firmly against the basin— you see that I do-therefore it is evident that the bottom of the vessel, as well as the end of the ruler, appears higher than it is in reality. In like manner, if we stand beside a river or pond, in which the water is so clear that we can see the bottom, it will, from the effect of refraction, appear much nearer to the eye than it is: so that, if the pond were six feet deep, we should suppose it to be only four feet and a half; that is, just one quarter shallower than the real depth. Many a poor lad has been drowned by venturing into water, which, owing to this deception, was much deeper than he had supposed. But on some occasions the refracting power of water has afforded a useful warning to persons who were aware of the fact, and prompt in taking advantage of it.

You have heard of coral rocks, and how they are built up from the depths of the ocean by

*Ih. p 37.

OF REFRACTION.

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the united labours of millions of little worms, till they reach the surface of the water. As these rocks rise almost perpendicularly, a ship may strike upon, or against them, before any change of soundings has put the mariner on his guard. Perhaps change of soundings is a phrase that some of you do not understand. Every ship requires a certain depth of water to swim in; therefore a man is stationed in the fore-part of the ship to find out the depth, by dropping a line, with a weight attached to it, down to the bottom of the sea. This operation is called sounding. On approaching common shores, the water becomes gradually shallower, and the pilot knows he must take care not to run the ship aground. But coral rocks give no warning: rising abruptly, even in the midst of deep water, the mariner may suddenly find himself in a very perilous situation. This accident happened to a ship which was pursuing its voyage in the sea of China, and inadvertently passed through a narrow opening in a reef of coral rocks, which rose beneath the water in the

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form of a horse-shoe, and surrounded the ship. When the alarm was given, the danger appeared imminent. The unusual transparency of the water afforded a distinct view of the enemy that lurked beneath; while its apparent depth was so lessened by refraction, that, as the waves swelled, the rocks seemed to be almost at the surface of the water; and the anchor, which had been let down on the first intimation of danger, appeared, like the rocks, to be lifted up. move forward, in any direction, threatened instant destruction. The only resource was to keep the vessel stationary, and send out boats to search, by sounding, for the passage through which they had entered. The narrow opening was by this means discovered; and the ship, safely traversing it a second time, was released from that terrible prison.*

Το

I have said that colour is not a quality of the object that seems to be tinged with it, but a property of the light which falls upon that object. A rose appears red because its petals reflect the

* Arnott's Elements of Physics, ii. p. 184.

red rays: a convolvulus reflects the blue rays. If it were possible for these flowers to grow and to bloom in darkness, the rose would not be red nor the convolvulus blue. A lettuce, when left to grow naturally, is green; but, as you may have observed, when the gardener ties it round with a piece of matting, the inner leaves, which compose the heart of the lettuce, continue to grow, but they are pale and colourless-deprived of air and light, they do not acquire the power of reflecting the green rays. Red rays proceed from the rose to our eyes, and we call the rose, red; but the redness is in the light.

You have seen, by the little experiment we have just tried, that our idea of the form of an object is influenced by the direction of the rays proceeding from it. That ruler appears bent or broken while the end of it remains in the water; though it is not the ruler that is bent, but the rays of light from which we receive our notion of the shape of the ruler.

This, then, is REFRACTION. I hope, when we

have occasion to make use of that word, you will distinctly recollect its meaning.

I will now endeavour to explain another change in the direction of rays of light, which is called REFLECTION. This word also is borrowed from the Latin :-flecto means, I bend. Reflected rays are those which, after striking or shining on a substance, are bent or driven back again.

When we see the reflection of ourselves in a looking-glass, the image in the glass is formed by the rays of light which fall upon our faces, and are reflected from our faces to the glass opposite to us. The rays cannot pass onwards, because the coating of tin-foil and quicksilver on the back of the glass prevents it: they are consequently reflected back again, from the mirror to our eyes, and, passing through the pupil, they form a corresponding image on part of the internal eye, which is so constructed as to reflect any object placed before it. The mind perceives that image or reflection, and then we say that we see the object.