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., — this will not do after all.They were eventually driven to this conclusion by an experiment of Fresnel's, towards which some preliminary work had however been done before, by the Jesuit Grimaldi among others.Figure IVdFresnel's experiment shook the corpuscular theory very considerably.His experiments are indeed most interesting, and we must try to get a clear idea of what is really happening when experiments are set up in the way he did.I beg you now, pay very careful attention to the pure facts; we want to study such a phenomenon quite exactly.Suppose I have two mirrors and a source of light — a flame for instance, shedding its light from here ().If I then put up a screen — say, here — I shall get pictures by means of the one mirror and also pictures by means of the other mirror.Such is the distribution you are to assume; I draw it in cross-section.Here are two looking-glasses — plane mirrors, set at a very small angle to one another, — here is a source of light, I will call it L, and here a screen.The light is reflected and falls on to the screen; so then I can illumine the screen with the reflected light.For if I let the light strike here, with the help of this mirror I can illumine this part of the screen, making it lighter here than in the surrounding region.Now I have here a second mirror, by which the light is reflected a little differently.Part of the cone of light, as reflected from here below (from the second mirror) on to the screen, still falls into the upper part.The inclination of the two mirrors is such that the screen is lighted up both by reflection from the upper mirror and by reflection from the lower.It will then be as though the screen were being illumined from two different places.Now suppose a physicist, witnessing this experiment, were thinking in Newton's way.He would argue: There is the source of light.It bombards the first mirror, hurling its little cannon-balls in this direction.After recoiling from the mirror they reach the screen and light it up.Meanwhile, the others are recoiling from the lower mirror, for many of them go in that direction also.It will be very much lighter on the screen when there are two mirrors than when there is only one.Therefore if I remove the second mirror the screen will surely be less illumined by reflected light than when the two mirrors are there.So would our physicist argue, although admittedly one rather devastating thought might occur to him, for surely while these little bodies are going on their way after reflection, the others are on their downward journey (see the figure).Why then the latter should not hit the former and drive them from their course, is difficult to see.Nay, altogether, in the textbooks you will find the prettiest accounts of what is happening according to the wave-theory, but while these things are calculated very neatly, one cannot but reflect that no one ever figures out, when one wave rushes criss-cross through the other, how can this simply pass unnoticed?Now let us try to grasp what happens in reality in this experiment.Suppose that this is the one stream of light.It is thrown by reflection across here, but now the other stream of light arrives here and encounters it, — the phenomenon is undeniable.The two disturb each other.The one wants to rush on; the other gets in the way and, in consequence, extinguishes the light coming from the other side.In rushing through it extinguishes the light.Here therefore on the screen we do not get a lighting-up but in reality darkness is reflected across here.So we here get an element of darkness ().But now all this is not at rest, — it is in constant movement.What has here been disturbed, goes on.Here, so to speak, a hole has arisen in the light.The light rushed through; a hole was made, appearing dark.And as an outcome of this “hole”, the next body-of-light will go through all the more easily and alongside the darkness you will have a patch of light so much the lighter.The next thing to happen, one step further on, is that once more a little cylinder of light from above impinges on a light place, again extinguishes the latter, and so evokes another element of darkness.And as the darkness in its turn has thus moved on another step, here once again the light is able to get through more easily.We get the pattern of a lattice, moving on from step to step.Turn by turn, the light from above can get through and extinguishes the other, producing darkness, once again, and this moves on from step to step.Here then we must obtain an alternation of light and dark, because the upper light goes through the lower and in so doing makes a lattice work.Figure IVeThis is what I was asking you most thoroughly to think of; you should be able to follow in your thought, how such a lattice arises.You will have alternating patches of light and dark, inasmuch as light here rushes into light.When one light rushes into another the light is cancelled — turned to darkness.The fact that such a lattice arises is to be explained by the particular arrangement we have made with these two mirrors.The velocity of light — nay, altogether what arises here by way of differences in velocity of light, — is not of great significance.What I am trying to make clear is what here arises within the light itself by means of this apparatus, so that a lattice-work is reflected — light, dark, light, dark, and so on.Now yonder physicist — Fresnel himself, in fact — argued as follows: If light is a streaming of tiny corpuscular bodies, it goes without saying that the more bodies are being hurled in a particular direction, the lighter it must grow there, — or else one would have to assume that the one corpuscle eats up the other! The simple theory of corpuscular emanation will not explain this phenomenon of alternating light and dark.We have just seen how it is really to be explained.But it did not occur to the physicists to take the pure phenomenon as such, which is what one should do.Instead, and by analogy with certain other phenomena, they set to work to explain it in a materialistic way.Bombarding little balls of matter would no longer do.Therefore they said: Let us assume, not that the light is in itself a stream of fine substances, but that it is a movement in a very fine substantial medium — the “ether”.It is a movement in the ether.And, to begin with, they imagined that light is propagated through the ether in the same way as sound is through air (Euler for instance thought of it thus).If I call forth a sound, the sound is propagated through the air in such a way that if this is the place where the sound is evoked, the air in the immediate neighbourhood is, to begin with, compressed.Compressed air arises here.Now the compressed air presses in its turn on the adjoining air.It expands, momentarily producing in this neighbourhood a layer of attenuated air.Through these successions of compression and expansion, known as waves, we imagine sound to spread.To begin with, they assumed that waves of this kind are also kindled in the ether.However, there were phenomena at variance with this idea; so then they said to themselves: Light is indeed an undulatory movement, but the waves are of a different kind from those of sound.In sound there is compression here, then comes attenuation, and all this moves on.Such waves are “longitudinal”.For light, this notion will not do.In light, the particles of ether must be moving at right angles to the direction in which the light is being propagated.When, therefore, what we call a “ray of light” is rushing through the air — with a velocity, you will recall, of 300,000 kilometres a second — the tiny particles will always be vibrating at right angles to the direction in which the light is rushing [ Pobierz całość w formacie PDF ]