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The photographic news
- Bandzählung
- 29.1885
- Erscheinungsdatum
- 1885
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- Englisch
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- F 135
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- Hochschule für Grafik und Buchkunst Leipzig
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- Hochschule für Grafik und Buchkunst Leipzig
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- http://digital.slub-dresden.de/id1780948042-18850000
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- Bandzählung
- No. 1406, August 14, 1885
- Digitalisat
- SLUB Dresden
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- Ausgabe
- Parlamentsperiode
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- Wahlperiode
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Zeitschrift
The photographic news
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Band
Band 29.1885
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- Register Index III
- Ausgabe Ausgabe 1
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- Ausgabe Ausgabe 817
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Band
Band 29.1885
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- Titel
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AvavsT 14, 1885.] THE PHOTOGRAPHIC NEWS 523 million times in a second. Therefore, you see, if you give an exposure of the 100th of a second you still have seven million millions of vibrations beating on the sensitive plate, so there is ample vibration to effect any chance on the molecule of silver chloride, supposing always the amplitude (or distance of swing) is sufficient. Instantaneous photography will not be complete, I suppose, until you can reduce by a million times. We may take it that an atom vibrates somewhat in the same way that a pendulum vibrates. Here I have a very rough contrivance to show what I mean. I set the pendulum swinging. Now picture to yourself that the bob is an atom, and picture to yourself, also, a wave of light falling upon that pendulum; if the wave of light be synchronous with the pendulum, it will increase the swing, or, in other words, it will increase the amplitude of the swing of the pendulum. For a rude illustration, suppose I take puffs of my breath as illustrating the beating of the wave of light, and suppose the atom to be at rest; I begin, and I blow ; every time 1 give a well-timed puff to that pendulum, the pendulum increases in amplitude, or swing. But if my breath does not come in unison with that pendulum [blowing irregularly], you see that very soon I should bring that pendulum to rest ; in other words, unless the wave of light beats in unison with the atom, the amplitude cannot be much increased. It is true that as long as the breath strikes the bob as it is going away from me the amplitude is increased ; but if the puffs are regular and slightly more rapid or slower than the pendulum oscillation, the amplitude must eventually be diminished. Here we are met with a difficulty, and a very great difficulty. I exposed the plate to the spectrum, and you see the blackening not only was where one wave of light synchronised with that atomic motion, but that there were a great many waves of light extending from the ultra-violet as far as the blue which affected it. How are we to get over that ? That is a difficulty which has puzzled a great many people. I would ask you again to form a mental picture of how that could possibly arise. I do not say that it is the correct way, but all I say is that you can form a picture in your own mind—can conceive of how it could be done. Here I have another pendulum, but in this case the bob is attached to an elastic band. The time of the vibration of a pendulum depends upon the length of the pendulum. Therefore, if, during the time of the oscillation of the pendulum, I alter the length, 1 also alter the rate at which the pendulum vibrates during any instant. I pull down the weight of the pendulum, and at the same time set it swinging, and you will see that during every part of this motion the length of the pendulum is altered so that a great many differently timed puffs of breath might be synchronous with the pendulum. It is not like this other rigid one, where it is of a definite length, but here the length of the pendulum keeps altering. I only ask you to form a mental picture of the way in which such a thing might happen. In this way you can picture to yourself how a molecule might vibrate, and still be synchronous with more than one vibration of light. Proceeding another step, I may say at once that, to my mind, the theory of the photographic image is well established. I know that there are some people who differ, but in my own mind the formation of the photographic image is not a working hypothesis, but it is a theory. The difference between a working hypothesis and a theory is this—that you adopt a certain idea, and say “I will work upon that idea, and see whether every experiment fits with the idea I have conceived. If it does not fit, then that working hypothesis is no use. I must give it up; I must take some other working hypothesis.'’ As regards the idea of the formation of the photographic image, I think that it has passed from the stage of the working hypo thesis into one of a really acceptable theory. It does not follow that everybody will accept it, but still it is an acceptable theory, accepted by most people. I am not going to enter into that very strongly to-night. At the next meeting of the Photo graphic Society, I propose to deal with it more fully; but, at the same time, I just wish to state publicly, to perhaps a more extended audience than I shall see at the Photographic Society in about three weeks’ time, that this photographic image theory —that is to say, the theory as to the action of light upon mole cules of silver—is as well established as, at all events, the wave theory of light itself. • Now I am going to show you an experiment which, perhaps, will help to illustrate what I mean by the vibrations of atoms. In this slide I have got a gelatine plate, and I have a little flat iron which has been made warm. It is rather too warm to be borne comfortably. Here I have a phosphorescent plate, which I propose to illuminate with magnesium wire, in order to give an even source of light; I press this flat iron against the back of the plate which is in this slide for a short time. I shall not let the plate coo], but while it is warm I will expose it to the phosphorescent light for about fifteen seconds. The plate is now allowed to become cold, and is developed. If everything has gone right, we ought to have something which shows us that the oscillations of the atoms of bromide of silver (which is the silver salt on this plate) have been given extra amplitude by the action of the heated iron to the back of the plate. I am afraid that I cannot show you the development in the light. [When the development had been carried out, the plate was shown.] You now see we have a picture of this flat iron produced by the deeper blackening of the heated part, though the whole plate was given but a short exposure to the light from the phos phorescent plate. I will impress this further upon you. I have here a collodio-bromide emulsion plate. But in this case, instead of heating it by a flat iron, we will heat it by immersion in hot water. Of course a collodion plate is not so sensitive as a gelatine plate. I put it into cold water for a short time to moisten it, and then dip half of it into some nearly boiling water; on withdrawing it, I expose it to this candle, and develop it when it gets cool, which we effect by placing it a short time in cold water. It will be seen that the part immersed in hot water is much blacker than that which was exposed cool. If I heat the plate and allow it to cool, and then expose, there will be no effect. The plate will develop normally, for the increased amplitude of vibration will have ceased, and the light will have to perform the same work on each part of the plate. Now, in whatever manner increased amplitude is given, when the cause of the increased amplitude is withdrawn, the amplitude will cease in the same manner. The case before us next was the cause, and it will cease after a very short period—in other words, when the plate gets cold. One of the chief reasons against what we maycall the “vibration theory” of the pbotographic image— namely, that the molecule is unaltered by the action of light - is this: that the increased amplitude would cease with the same rapidity with which it would cease when the hot iron was applied to the back ; that is to say, after five or ten minutes the amplitude of the vibrations would come back to the normal extent, a condition which is not fulfilled in the photographic image. I can illustrate this in a very visible manner. I think you can all see this phosphorescent plate. Now, what is the reason of that phosphorescence taking place ? It is that the atoms of the molecules which comprise this phosphorescent material are swinging in a certain rhythm, which gives us the sensation of light. Now, if I apply a hot iron to the back of this plate, I think at once you will see that the image of the hot iron is pre sent. Here is the same kind of action taking place in the one case as in the other. Now we come to another point, which is a slightly different one, and that is the energy of radiation. I may say that the energy of radiation is a subject on which I could discourse for a good many hours, but here I can devote but two minutes to it. I must try to make it as clear as I can. I hold in my hand a little instrument which is called a thermopile, which you see has a narrow slit which could be narrowed to any degree of fineness ; attached to it is a screw motion, which will make that slit travel along the base of the instrument; beneath that slit are some thermo-electric couples. It is not my business to enter into how they are made, but still we know that, when thermo-electric couples are heated, an electric current is gene rated sufficiently strong to cause the needle of a galvanometer to deviate ; and the amount of energy of radiation which falls upon the face of the pile can be measured by the deviation of the galvanometer needle, from the energy heating the lampblack at the junction of the couples. In a great many experiments which were made, this thermopile was caused to travel along the spectrum by the screw motion, and at every part of the spectrum at distances of, say, a quarter of a turn or half a turn of the screw, the amount of deviation which was given to the galvano meter needle was read off. By that means we are able to com pare the energy existent at different parts of the spectrum. The spectrum used was that of the electric light, the compara tive energies at different parts of its spectrum I have in the diagram—at five turns of the screw we have the end of the red, and at different turns we have the yellow, the green, the blue, and the violet; whilst from five to twenty we have the dark rays which lie below the red, and with which we are not to deal
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