pendicular axis the cylinder, D, driven by a grooved pulley and cords, E E, from a double system of multiplying wheels, so as to give swift rotation. This cylinder is painted with uranium or canary-glass, powdered, and the powder laid on as paint with some transparent vehicle. Turning the slit and cylinder towards the observer, it will be obvious that if there were no duration of luminosity, or true "phosphorescence," in the case of the fluorescent cylinder, it must appear dark; but on imparting rotation, it shines brilliantly with the characteristic green light. Some of the powerful fluorescent dyes already mentioned give the same phenomena. Thus, then, fluorescence is linked on to phosphorescence; and though all fluorescent substances will not show this with ordinary experimental means, there can be little doubt that it is only a question of degree, and of powers of observation.

Once again, therefore, Light has revealed to us the minute, invisible motions which its own ether-vibrations communicate to the molecules of bodies. Where we may have thought all was still, it shows us molecules in constant and rapid motion. Where we seem to have lost that motion, further reflection and experiment yield us but another and impressive proof of the great law of the Conservation of Energy. We see that no motion is destroyed; but that every single movement does its work, and is converted into some other form. thus get a very vivid idea of the intense reality of these motions, which seem hypothetical only because they elude the direct examination of our senses. That sense of their reality and definiteness will help us to understand the beautiful and new field of experiment, embracing the most splendid phenomena of physical optics, which we now have to investigate.

We

CHAPTER IX

INTERFERENCE

Net Result of Two Different Forces-Liquid and Tidal Waves-Why Single Interferences are not Traceable in Light-Interference of Sound Waves-Thin Films of Turpentine, Transparent Oxide, Soap, Water, and Air-Colour Dependent on Thickness of the Film-Newton's Rings—Proved to be Dependent also on Reflection from both Surfaces -Spectrum Analysis of Films—Phenomena of Thicker Films-SoapFilms and Sound Vibrations-Colours of Thick Plates-Fresnel's Mirrors - Fresnel's Prism - Irregular Refraction-Diffraction-Gratings-Telescopic Effects-Other Simple Experiments in DiffractionStriated Surfaces-Barton's Buttons-The Diffraction Spectrum— Measurement of Waves-Change of Phase in Reflection from a Rarer Medium - Photographic Demonstrations of Interference — Hertz's Experiments-The Size of Molecules of Matter-Appendix on Diffraction in the Microscope.

98. Net Result of Two Different Forces or Motions. We have now to study a class of experiments which most of all clearly demonstrate the wave character of the phenomena which constitute Light. We know that different separate motions can so act upon the same particle of matter, as either to combine and strengthen, or to neutralize and destroy each other; because the actual motion of any particle must result from the net sum, difference, or other result of the forces which act upon it. Take a billiard ball travelling in a direction and at a rate resulting from some stroke of the cue

e;

if we impart another impulse in the same direction the velocity will be increased; while if the ball be met by a second force of the same amount, it is brought to a standstill.

99. Interference of Liquid Waves.-The same must result in the case of any series of vibrations of equal amplitudes

and periods, such as constitute a wave. If we drop two stones at some distance apart into the same pond, the circular waves from one will cross those from the other. At some

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INTERFERENCE OF WAVES

149

points the crests will coincide, and reinforce each other's upward movements; at others the same particle of water is elevated by one wave and depressed by the other; there it is at rest. The consequence is a beautiful pattern caused by the intersecting ripples. Fig. 97 shows such a pattern caused in an elliptical bath of mercury by a drop or point introduced at one of the foci. They can be shown by the vertical attachment (§ 12) to the lantern, laying over the condenser a glass plate to which is cemented an elliptical tin wall, making a tank some 6 inches in diameter and an inch deep, with a glass bottom. On focusing the surface, and then exciting waves by the point of a rapidly vibrating wire, the intersections of the original and reflected waves will be depicted upon the screen.

100. Interference of Tidal Waves.-The same thing is true of tidal waves, a remarkable example of which is found in the channel between England and Ireland. The flood-tide, sweeping round from the Atlantic to the north and south of Ireland, meets about a line which usually passes just across the south of the Isle of Man. There the two currents destroy one another, and there is practically none, while the rise and fall of the tide is greatest. But going back from this point to north and south, there are also two points (near Portrush, in Antrim, at the north of the Irish Channel; and near Courtown, in Wexford, at the south) where the falling tide meets the next rising tide; at these points, therefore, there is practically no rise or fall of tide whatever, while the current is at the maximum. The same is true of the vast tidal waves that sweep round the globe. At certain times the sun-wave coincides with the moon-wave, and then we have the greatest tidal motion; at others the sun's wave opposes the moon's wave, and we have the least motion.

IOI. Single Interferences not Traceable in Light and Sound. But here we must make a very important distinction, the want of which has caused many a student difficulty. In the foregoing cases we could trace the interferences of single

waves, because their motions were large, occupied considerable time, and thus enabled us to trace them---most clearly in the grand tidal waves, which are longest of all. The student is apt to fancy that, in a similar way, rays from any two points of light must be constantly destroying one another by interference, much as in Fig. 98, supposed to represent the rays from two lighthouses. And to some extent they undoubtedly do SO. But they can only thus act on each other at the points where the undulations cross; and in the case of light the vibrations are so enormously rapid and numerous, that

the comparatively few extinctions of this kind are not sensibly missed.

But if we bring a whole wave series to act upon another similar whole wave series, then any effect at one point in any waves of the series is repeated throughout the series, and the effect becomes visible. In the case of sound we can get similar wave series easily, by employing exact unisons; and so it will be found, if a tuning-fork be struck and held close to the ear, that on turning it round on the stem there is a position in which the sound is nearly or quite extinguished. This position differs, as it should do, with the key of the fork; but is when the two prongs are at an angle of nearly 45° with the direction of the ear. If the fork is steadily rotated, the sound will be alternately extinguished and reinforced, according to the phases in which the waves from each prong encounter one another.