CHAPTER XV.

MIRROR EXPERIMENT OF FRESNEL.

UNDULATORY MOVEMENT.

FIG. 133.

87. THE reader has hitherto had his attention confined to the experimental investigation of the laws of the phenomena of light without speculating as to what light essentially is. A series of phenomena now present themselves which raise again this question of the nature of light, and at the same time afford the means of answering it. Let two mirrors, A Band BC (fig. 133), be made of black glass and be so placed as to meet at the vertical slit, B, the one, B C, being permanently fixed in a wooden frame

Fresnel's mirror.

(Holzklötzchen) which can be moved along a vertical rod and fastened by a wooden screw T, whilst the other, A B, is revolvable by means of the screw 8 around the angle B by means of the hinge attached to it. The moveable mirror is to be placed in such a position that

its plane forms a very obtuse angle (not differing much from 180°) with that of the fixed mirror.

A sharply defined point of light is required, and may be obtained by letting the solar rays proceeding from a Heliostat fall upon a lens (fig. 134) of short focal distance, which unites them into a focus P. The luminous point P emits rays which strike both mirrors; from the mirror A B they are so reflected that they

appear as if they came from the image-point M of this mirror. The mirror B C, on the other hand, reflects the rays as if they proceeded from its image-point N. In order that the two mirrors may each have only one reflecting surface and have only one image-point, they must be made of black glass or of metal.

From these mirrors two cones of light Mm m' and Nn n' are obtained, which appear to proceed from the points M and N. They have the space Bm n (shaded in the figure) common to both, so that the field between

m and n upon the screen m'n' situated in the path of the reflected ray receives light simultaneously from the two cones of light. In this middle area a series of vertical dark lines are perceived, but if one of the glasses be covered the lines immediately vanish and the area which now receives only the light from the opposite mirror appears to be uniformly illuminated throughout its whole extent. The lines however immediately reappear if the cover be removed, and to the light proceeding to the screen from the point M is added that also which proceeds from the point N.

It has thus been demonstrated that light added to light may, under certain circumstances, cause darkness.

If by turning the screw S (fig. 133) the angle of the two mirrors be made less obtuse, the lines become narrower and closer together till they ultimately become so fine that they can no longer be distinguished. Hence to render the lines distinctly perceptible the angle between the two mirrors must be very obtuse, or what comes to the same thing, the mirror images M and N must be very closely approximated.

Instead of making the experiment with a screen so that many can see it at the same time, any individual may observe it directly by making his retina take the place of the screen. This subjective method of observation has the advantage that a feeble source of light may be employed; and then, if the homogeneous light of the Sodium flame be used, the entire field of vision may be observed to be filled with numerous vertical and completely black lines.

88. The just-described mirror experiment of Fresnel, named after the genial physicist who conceived it, teaches that light combined with light may, under

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certain circumstances, produce darkness. What then must be understood by the term 'light,' to enable this apparent paradox to be explained?

This much is certain, that every luminous body must be regarded as the seat of a motion which is by some means propagated to our optic nerves and arouses in them the sensation of brightness.

Two modes, however, are only known in which movement may be propagated from one point of space to another.

The first mode is the immediate transference of motion in which the moved body itself or parts of the same traverse the space between the two points, as when a cannon ball flies to its goal from the cannon.

The second mode of transference takes place mediately through an elastic medium intervening between the two points, in which medium the body originally in motion excites a vibratory movement that is propagated from particle to particle, it may be to a great distance, without a particle of the originally moving body itself or any portion of the propagating medium moving from its original position to any considerable extent. This process is called undulatory movement.

As an example of the former, the sense of smell may be taken, which is excited by the immediate transference of particles of the odorous material to the olfactory organ. If a flask containing some ammoniacal gas, which is colourless, be opened, those near it quickly perceive the stimulating odour of the gas, whilst it is only perceived by those who are more distant after the lapse of some time. It would be easy to demonstrate by appropriate tests the presence of particles of ammonia even in the furthest corner of a

room. The smell is perceived still more strongly if a second flask be opened, so that the number of particles of ammonia present in the air is increased; it would, however, be needless to do this, since all must be satisfied that the sense of smell is excited by particles of the odorous material which come into direct contact with the olfactory organ, and that by increase of the effective particles alone can the intensity of the sensation be augmented.

Another of our senses, hearing, on the other hand, receives its impressions through the second mode of propagation, since every resounding body puts the air around it into undulatory movement. If a bell be struck its sound is heard simultaneously with the blow. The blow makes the bell vibrate, that is to say, causes its particles to make rapid to and fro movements or vibrations which are felt by the hand in contact with it as a trembling. The vibration communicates itself in the first instance to the particles of air in immediate contact with the bell, and as these move to and fro in the same rapid manner they produce the same effect upon the particles of the next adjacent layer of air as the bell itself, and set them in motion. In this way the vibratory movement is propagated with great rapidity from one layer of air to another, and finally, on reaching the ear, excites in the auditory nerve the sensation of sound. But it is certain that neither particles of the bell itself, nor even particles of the air immediately surrounding the bell, penetrate the ear; if they did, as sound travels at the rate of 1,116 feet in the second, they would strike on the tympanum with a velocity exceeding that of the most violent hurricane. An extremely simple experiment may now be considered,