It is obvious that when rolled along a smooth board such an axle will preserve one direction. But referring now to Fig. 79, let there be glued on the board a rectangular and a triangular piece of the thick pile plush called "imitation sealskin." If this is presented to the wheel-track the right

HA

FIG. 79.

way of the pile, it will retard the motion considerably, and when the track A B is oblique, the wheel that first meets the plush being first hindered, the track will swing round to the

direction B C, and on leaving the plush resume the track C D, exactly picturing the course of a ray of light through a thick piece of glass. The triangular piece in the same way represents a prism, the track E F being refracted to F G, and thence to G H. Nay, even dispersion may be thus pictured

by having a second pair of wheels, s (Fig. 80), of smaller diameter, to represent smaller waves— -say 11 inches to 1 inches. These wheels will be found perceptibly more deflected from S F to the track F K. The wheels may either roll freely down a slight incline, or may be held back by a thread at the centre of the axle. Finally, total reflection may be illustrated as in Fig. 81, for it will be found that

FIG. 81.

if the track A B leaves the edge E F of the velvet at a certain angle, the wheel c, which first emerges, gains so much on the one still upon the velvet, that the axle swings right round and proceeds on the track B D.

These experimental illustrations will sufficiently enable us to grasp vividly all the main points of the wave theory. We shall now resume the experimental study of colour.

CHAPTER VI.

COLOUR.

Absorption of Colours-What it means-Absorbed, Reflected, and Transmitted Colours-Complementary Colours-The Eye cannot judge of Colour-waves-Mixtures of Lights and Pigments, and their Difference-Primary Colour Sensations-Not the same thing as Primary Colours-Colour as we see it only a Sensation-Experiments showing merely sensational Colour,

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We have now cleared the way for another class of experiments, for which, to work with comfort, we must somewhat alter our arrangements by removing the objective, and placing on the flange-nozzle of the lantern a black card or other cap with a perpendicular slit cut in it rather longer than we have hitherto worked with-sayinch to inch wide by 1 inches long. A long slit, owing to the convergence of rays by the lens, gives a perceptible curvature across the spectrum band; but this need not matter to us. Arrange the loose focusing lens F (the longest-focus one if there are two) so as to focus the slit on the screen if the beam were direct (the lantern must, of course, be deflected, as for all prism work), and adjust the prism P otherwise as before. The whole arrangement is shown in Fig. 82, and its object is simply to produce on the screen the spectrum

A brass cap with adjustable slit is, of course, much more convenient.

ot a slit upon which we can more readily make various experiments.

69. Absorption of Colours.-Providing now some coloured glasses, or some strips of coloured gelatine between glass plates, we make some experiments which teach us a very important lesson. We are apt to think that the sunlight which comes through a red glass window is all turned into red-made red. Well, there is the spectrum of our complete or white light on the screen, drawn out into its constituent colours. Over half the slit hold a bit of the red

glass; if the light, or most of it, is really "reddened," all the spectrum ought to be turned into red. It is no such thing, however. There is no colour in the spectrum of the glass where that colour does not exist in the ordinary spectrum; the sole effect is that certain colours are cut out, or absent. We get the colour, as so often before, by suppressing colour. If the glass is a pretty pure red, only red and a little orange, A B, is seen in the spectrum of the half slit covered by the glass; all the rest is cut away. So of all

the other gelatines or glasses, but we soon find it is very difficult to find a pure colour; generally there are left, at least, two well-marked colours; and if we unite just those portions of the ordinary spectrum, by employing proper slits in proper places, and uniting the colour passing through them by our confectioner's jar, or a cylindrical lens (§ 49) we get the same colour as the coloured glass.

'We see, therefore, that in passing through a transparent body, its molecules take up or absorb the waves of certain periods, and the remainder passing through give the colour of the glass. This is not at all difficult to understand. We have supposed every matter-molecule to have its own period of vibration (see next chapter); or perhaps more often periods, as it seems probable most molecules are complex. These molecules can freely communicate any vibrations to the ether atoms; but conversely it must be different: the matter-molecules can only take up synchronous vibrations. That one tuning-fork will communicate its vibrations to another of the same note we know; and we also find that in thus giving up its motion to the second, it loses its own more quickly than one that does not. A fork mounted on a unisonal resonance-case sounds louder, but stops sooner, than one unmounted: it has been imparting its motion to the unisonal column of air, and in so doing exhausts its energy. See then what must happen. If waves which produce red sensations require vibrations of 450 million millions of times in a second, and such rays pass with others through matter whose molecules vibrate at that rate if set in motion, these must themselves take up or absorb all such from the ether, and the rest passing through give a colour due to the other rays-in this case green.

We shall examine tests of this theory presently; but meantime, however absorption is produced, many other experiments will show that the colour of our glass is

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