The point about N-rays is that they don't exist. They attracted a lot of attention after being 'discovered' in 1903 as a new form of radiation by a French scientist, Prosper-René Blondlot. However a sceptical American scientist, on a visit to Blondlot's laboratory, covertly established that Blondlot was seeing something that wasn't there. This was one of the examples of what Irving Langmuir dubbed 'pathological science', a process in which people are 'tricked into false results ... by subjective effects, wishful thinking or threshold interactions'.
This is the relevant extract from Langmuir's talk (a complete transcript can be read here):
'In 1903, Blondlot, who was a well-thought-of French scientist, member of the Academy of Sciences, was experimenting with x-rays as almost everybody was in those days, The effect that he observed was something of this sort. I won't give the whole of it, I'll just give a few outstanding points. He found that if you have a hot wire, a platinum wire, or a Nerns't filament or anything that's heated very hot inside an iron tube and you have a window cut in it and you have a piece of aluminum about 1/8 of an inch thick on it, that some rays come out through that aluminum window. Oh, it can be as much as two or three inches thick and go through aluminum, these rays can, but not through iron. The rays that come out of this little window fall on a faintly illuminated object, so that you can just barely see it. You must sit in a dark room for a long time and he used a calcium sulfide screen which can be illuminated with light and gave out a very faint glow which could be seen in a dark room. Or he used a source of light from a lamp shining through a pinhole and maybe through another pinhole so as to get a faint light on a white surface that was just barely visible.
'Now he found that if you turn this lamp on so that these rays that come out of this little aluminum slit would fall on this piece of paper that you are looking at, you could see it much better. Oh, much better, and therefore you could tell whether the rays would go through or not. He said later that a great deal of skill is needed. He said you mustn't ever look at the source. You don't look directly at it. He said that would tire your eyes. Look away from it, and he said pretty soon you'll see it, or you don't see it, depending on whether the N-rays are shining on this piece of paper. In that way, you can detect whether or not the N-rays are acting.
'Well, he found that N-rays could be stored up in things. For example, you could take a brick. He found that N-rays would go through black paper and would go through aluminum. So he took some black paper and wrapped a brick up in it and put it out in the street and let the sun shine through the black paper into the brick and then he found that the brick would store N-rays and give off the N-rays even with the black paper on it. He would bring it into the laboratory and you then hold that near the piece of paper that you're looking at, faintly illuminated, and you can see it much more accurately. Much better, if the N-rays are there, but not if it's too far away. Then, he would have very faint strips of phosphorescent paint and would let a beam of N-rays from two slits come over and he would find exactly where this thing intensified its beam.
'Well, you'd think he'd make such experiments as this. To see if with ten bricks you got a stronger effect than you did with one. No, not at all. He didn't get any stronger effect. It didn't do any good to increase the intensity of the light. You had to depend upon whether you could see it or whether you couldn't see it. And there, the N-rays were very important.
'Now, a little later, he found that many kinds of things gave off N- rays. A human being gave off N-rays, for example. If someone else came into the room, then you probably could see it. He also found that if someone made a loud noise that would spoil the effect. You had to be silent. Heat, however, increased the effect, radiant heat. Yet that wasn't N-rays itself. N-rays were not heat because heat wouldn't go through aluminum. Now he found a very interesting thing about it was that if you take the brick that's giving off N-rays and hold it close to your head it goes (p.5) through your skull and it allows you to see the paper better. Or you can hold the brick near the paper, that's all right too.
'Now he found that there were some other things that were like negative N-rays. He called them N'-rays. The effect of the N'-rays is to decrease the visibility of a faintly illuminated slit. That works too, but only if the angle of incidence is right. If you look at it tangentially you find that the thing in-creases the intensity when you look at it from this point of view. It decreases if you look at it normally and it increases if you look at it tangentially. All of which is very interesting. And he published many papers on it. One right after the other and other people did too, confirming Blondlot's results. And there were lots of papers published and at one time about half of them that were confirming' the results of Blondlot. You see, N-rays ought to be important because x-rays were known to be important and alpha rays were, and N-rays were somewhere in between so N-rays must be very important. (Laughter)
'Well, R. W. Wood heard about these experiments--everybody did more or less. So R. W. Wood went over there and at that time Blondlot had a prism, quite a large prism of aluminum, with a 60o angle and he had a Nernst filament with a little slit about 2 mm wide. There were two slits, 2 mm wide each. This beam fell on the prism and was refracted and he measured the refractive index to three significant figures. He found that it wasn't monochromatic, that there were several different components to the N-rays and he found different refractive indices for each of these components. He could measure three or four different refractive indices each to two or three significant figures, and he was repeating some of these and showing how accurately they were repeatable, showing it to R. W. Wood in this dark room.
'Well, after this had gone on for quite a while, and Wood found that he was checking these results very accurately, measuring the position of the little piece of paper within a tenth of a millimeter although the slits were 2 mm wide, and Wood asked him about that. He said, "How? How could you, from just the optics of the thing, with slits two millimeters wide, how do you get a beam so fine that you can detect its position within a tenth of a millimeter?"
'Blondlot said, "That's one of the fascinating things about the N-rays. They don't follow the ordinary laws of science that you ordinarily think of." He said, "You have to consider these things all by themselves. They are very interesting, but you have to discover the laws that govern them."
'Well, in the meantime, the room being very dark, Wood asked him to repeat some of these measurements which he was only too glad to do. But in the meantime, R. W. Wood put the prism in his pocket and the results checked perfectly with what he had before. (Laughter) Well, Wood rather cruelly published that.(6, 7) And that was the end of Blondlot.
'Nobody accounts for by what methods he could reproduce those results to a tenth of a millimeter. Wood said that he seemed to be able to do it but no-body understands that. Nobody understands lots of things. But some of the Germans came out later--Pringsheim was one of them--came out with an extremely interesting story. They had tried to repeat some of Blondlot's experiments and had found this. One of the experiments was to have a very faint source of light on a screen of paper and to make sure that you are seeing the screen of paper you hold your hand up like this and move it back and forth. And if you can see your hand move back and forth then you know it ie illuminated. One of the experiments that Blondlot made was that the experiment was made much better if you had some N-rays falling on the piece of paper. Pringeheim was repeating these in Germany and he found that if you didn't know where the paper was, whether it was here or here (in front or behind your hand), it worked just as well. That is, you could see your hand just as well if you held it back of the paper as if you held it in front of it. Which is the natural thing, because this is a threshold phenomenon. And a threshold phenomenon means that you don't know, you really don't know, whether you are seeing it or not. But if you have your hand there, well, of course, you see your hand because you know your hand's there, and that's just enough to win you over to where you know that you see it. But you know it just as well if the paper happens to be in front of your hand instead of in back of your hand, because you don't know where the paper is but you do know where your hand is. (Laughter) '