The rays were a year old before Wood went to Nancy, and in that year the people who could not see them had already said why. Their explanation was not that Blondlot was dishonest. It was that an eye in the dark does, by itself, exactly what he was asking it to report.
Lummer, 27 November 1903
Otto Lummer, of the Physikalisch-Technische Reichsanstalt in Berlin, read a paper to the German Physical Society on 27 November 1903 that Nature printed in translation on 18 February 1904 under the heading “M. Blondlot’s n-Ray Experiments”. He began by granting the facts as Blondlot stated them: rays said to come from an Auer burner or a 200-watt Nernst lamp, to pass through aluminium, to be stopped by “the slightest film of water”, to be absorbed by cold platinum and to pierce red-hot platinum. Then the state of the evidence. “Although the change in brightness is said to be considerable, neither Blondlot, Rubens nor others have hitherto succeeded in demonstrating objectively the corresponding transformation of energy. At the same time the phenomena observed subjectively by Blondlot could not be perceived by Rubens and others in repeating the latest experiments with slightly illuminated or phosphorescent surfaces.”
His claim was that “a whole set of Blondlot’s experiments may be almost exactly imitated in their effects without employing any source of illumination whatever”, and that the changes “may be referred to processes taking place in the eye itself, and, in fact, to the contest between the rods and cones of the retina in seeing in the dark.”
The physiology was then new. By the work of König and of von Kries, the retina had two kinds of receptor: the cones, which “render vision possible in a very bright light” and give colour, and the rods, which “are totally colour blind, and only come into action in a very dim light, and are endowed with the property of considerably increasing their sensitiveness in the dark”. The centre of the retina, the fovea, “contains cones only and no rods”. So a very faint object is seen only by the rods, off to one side, and when the observer turns to look straight at it the fovea “sends no message of light to the brain; accordingly we cannot see the spot gazed at.” Lummer had shown this himself with a platinum plate warmed in the dark: at about 400 degrees it appears as a colourless “grey glow” that vanishes when stared at, and only above 500 degrees does it become a “red glow” the cones can see. “Thereby we are confronted by the remarkable fact that we see something which we are not gazing at, whilst it becomes invisible when we wish to fix our eyes upon it.”
Then the application. In Blondlot’s experiments “one perceives a very small slightly luminous surface, e.g. a dull red glowing platinum plate, in the dark and fixes one’s gaze upon it. Before bringing one’s undivided attention to bear on it, it is seen by the extra-foveal parts of the retina, because the eye involuntarily endeavours to gather as much light as possible, thus consequently both rods and cones take part in the vision. As soon, however, as the lead screen or the hand is interposed between the source of illumination and the luminous platinum surface, the observer, in order to see the change in it, will fix his gaze as directly as possible on the platinum plate, thereby excluding the rods. The necessary consequence will be that the platinum plate will appear reddish and less bright, and the rod-white of the peripheral parts of the retina be lost. But this fixing of the gaze requires time and effort. The darkening and the red colouring observed will also require a certain amount of time, and as soon as the hand or the screen is removed the eye will return as quickly as possible to extra-foveal observation, in which it receives more light.”
He then quoted Blondlot’s note of 2 November 1903 word for word, in French and in English, for the match: the strip of paper that grows dark and loses its outline when a lead plate or the hand is interposed; the image in the knitting needle that “becomes dark and reddish”; and the statement that “all these actions of the n-rays on light require an appreciable time for their production and disappearance”. Rubens, he added, had told him that he observed by gazing steadily at the surface, “and could perceive no brightening even when a very powerful Nernst lamp was employed.” Lummer did not say the rays did not exist. He said that Blondlot “does not state with what visual apparatus one ought to observe, nor does he give warning of the illusions one may fall into in carrying out his experiments”, and that “as soon as the phenomena observed by M. Blondlot shall have been incontestably proved by means of objective instruments of precision, these few remarks on the n-rays will be only of secondary importance.”
Brown, Burke and McKendrick
In London, S. G. Brown wrote to Nature in January 1904 that he had independently found a faint zinc sulphide screen brighten when brought near the body, had taken it for “an undiscovered ray given off by the flesh”, and had then found it was warmth: a fluted jar of warm water held against the back of the screen printed its pattern on the glow. John Butler Burke at the Cavendish could get nothing from calcium sulphide screens of Blondlot’s dimensions, with a lead screen or the hand interposed, or with a quartz lens, and was “at a loss to find any other explanation of M. Blondlot’s results than that he has come across a radiation to which some men are blind and others not so.” In June, after trying the photographs, he wrote that he had met nobody “who, on severe cross-examination in the dark, did not satisfy me that the variations in brightness which he had observed were altogether subjective, and the result of imagination or fatigue, for expectation counts for a great deal in these observations, and concentration of attention for still more.”
John G. McKendrick, the professor of physiology at Glasgow, did the experiment that Nancy never did, with seven observers, “five doctors, one student, and one laboratory attendant”, kept in an absolutely dark room and then shown calcium sulphide screens. “The observers were told first to look steadily at the screens and report any variation in brightness, calling out ‘bright,’ ‘dimmer,’ ‘dim,’ ‘brighter,’ &c., as the appearances seemed to change. Even with the screens on the slips of glass the observers after a few moments were able to call out the changes, although there was no attempt at muscular contraction.” When muscles were contracted behind the screen, sometimes after “bright” and sometimes after “dim”, “in the great majority of cases the effect we looked for did not follow.” Then the test that answers Blondlot’s instruction to see without looking: “We next told our observers to look, as it were, into the distance beyond the bright spot, and to report on the brightness of the screens. When the accommodation of the eyes for near vision was relaxed they reported without exception that the brightness of the screens was constant, and that muscular contraction made no difference.” And when they touched the backs of the screens, “thus warming them, they reported an increase of brightness.”
McKendrick’s own guess was that the eye could not hold a faint glowing spot in focus, so that “there is a wavering movement of the ciliary muscles, and probably also a wavering in the size of the pupils”. He did not claim to have explained Nancy. “How is it,” he asked, “that he and many of his compatriots see increase of brightness under conditions in which we see none?” The question was a fair one in April 1904, and it was answered in September, not by physiology but by a hand held still.