Wilhelm Röntgen and the Invisible Light
This story originally appeared in the Health Physics Society Newsletter.
Part One: X-rays
Had he been looking directly at the barium platinocyanide screen, he might not have seen its faint fluorescence. But Röntgen was looking elsewhere, and the light coming through the corner of his eye focused on the most sensitive part of the retina, the parafoveal region (Seliger 1995).1 This faint glimmer of fluorescence led Röntgen to the discovery of X-rays, also known as the "invisible light."
Invisible. X-rays could not be seen. Röntgen was quite clear about this: "The retina of the eye is insensitive to our rays; the eye brought close to the discharge apparatus registers nothing" (Röntgen 1895).
Nevertheless, it was only a matter of months before G. Brandes had proved him wrong: if sufficiently energetic, x-rays could produce a uniform blue-gray glow that seemed to originate within the eye itself!2
Röntgen soon confirmed the essential elements of Brandes’ observation, and reported the following incident in his third and final paper on X-rays: "In my record book there is also a note, written at the beginning of November 1895…[that while I was] in a completely darkened room near a wooden door, on the other side a Hittorf tube was placed, a feeble sensation of light that spread over the whole field of vision when discharges were sent through the tube. Since I observed this phenomenon only once, I thought it was subjective" (Röntgen 1897).3 Translation in plain English: "Okay, I said in my previous paper that I hadn’t seen anything. But I was just being cautious. I really had seen X-rays in November of 1895. So, no matter what Brandes thinks, I was the first."
The invisible light could be seen.
Part Two: Radium
The first to "see" the mysterious radiation from radium, or any other radioactive material for that matter, was Friedrich Giesel (1899a). Giesel, a German chemist who "made the commercial production of radium a kind of hobby," is sometimes credited with discovering radium independently of the Curies. As such, it is not surprising that Pierre Curie lost no time in reporting his own observations: "To obtain this effect, one places the box containing the radium in front of the closed eye or against the temple" and "one can attribute this phenomenon to a phosphorescence in the middle of the eye under the action of the invisible rays of radium" (Curie 1900, 1903).4
In the United States, the Colorado physician George Stover5 was among the first investigate radiation phosphenes (the proper name for visual sensations induced by radiation within the eye): "Sitting in a perfectly dark room and closing the eyes, if the tube of radium is brought close to the eyelids a sensation of light is distinctly perceived, which disappears on removal of the tube....Contrary to the statement of some observers, I have not been able to perceive the sensation of light when the tube is brought in contact with the rear of the head near the brain center of vision" (Landa 1987).
Part Three: Barium Platinocyanide
One of the oddities of fate is that Röntgen had the opportunity to "see" radium well before he ever saw x-rays - his barium platinocyanide screen, the instrument which revealed the existence of X-rays, was loaded with the stuff! Not only was radium present in the ores from which the screen’s barium was obtained, the two elements are so similar that they followed each other through the entire chemical extraction process.
This is not to say that there was enough radium for Röntgen to have detected a radiation phosphene had he placed his eye against the screen. But Röntgen still might have seen a glow of different sort if another observation by our old friend Friederich Giesel is to be believed: that because of its radium content, barium platinocyanide "phosphoresces by itself very strongly" (Giesel 1899b). If, as Giesel suggests, barium platinocyanide is in a constant state of self-induced fluorescence, even in the absence of X-rays, why hadn’t Röntgen observed this earlier?
Fortunately, I was in a position to see for myself since I had recently come into the possession of some 1890's-vintage barium platinocyanide—barium platinocyanide that once belonged to the great Robert Woods. To confirm Giesel’s claim, I sealed myself in a completely darkened room along with a vial of the barium platinocyanide (which had been shielded from light for the previous two weeks). After half an hour, enough time for my eyes to achieve maximum sensitivity, unable to tolerate the suspense any longer, and quivering with anticipation, I picked up the vial and slowly waved it back and forth. And out of the darkness it came… like an apparition from the past… fluorescence! The latter was barely visible, true, but there is little doubt that Röntgen’s barium platinocyanide screen, the world’s first radiation detector, was continuously and visibly responding to its own radioctivity.6
While it is not surprising that Röntgen never saw the faint fluorescent effects of the radium in the screen, he unknowingly came within a whisker of observing its photographic effects! According to Howard Seliger (1995), Röntgen had indicated an intention to apply barium platinocyanide against a photographic plate to intensify the photographic effect of the X-rays. But for some reason, he never carried out the experiment. As Seliger speculated (1995), if Röntgen had performed his intended experiment he might have observed that the barium platinocyanide created a photographic image similar to that in the accompanying figure—even without exposing the plate to x rays. And this image, an autoradiograph of barium platinocyanide, might have led Röntgen to the discoveries of radioactivity and radium—ahead of Becquerel and the Curies!
He could have been famous—really famous!
- Since the screen was some distance away, the solid angle subtended by the source at the eye was small. Had Röntgen been gazing directly at the screen, the image would have focused only on the fovea centralis, a rod-deficient and relatively insensitive region of the retina.
- The most likely explanation appears to be that the visual purple of the retina is directly excited by the X-rays. (Steidley 1990).
- Röntgen was color blind and never commented on the colors of the fluorescence he observed.
- The primary mechanism by which radium-related phosphenes occur seems to be through Cerenkov radiation induced by beta particles in the transparent media of the eye (Steidley 1990).
- A victim of "grossly excessive" radiation exposure, Stover philosophized just prior to his death, "A few dead or crippled scientists do not weigh much against a useful fact." He wrote the popular song On the Beach at Waikiki, and after his death his cremated remains would be scattered—you guessed it—on the beach at Waikiki (Landa 1987). More than one vacationing health physicist has probably had this radiation martyr’s ashes stuck to the bottom of his/her feet.
- The radium content of barium ore is extremely variable. Giesel’s barium might have come from the Erz mountain region where the radium content, and the resulting intensity of the fluorescence, would have been high.
- Röntgen, W. Further Observations on the Properties of X-rays. March 10, 1897. Translation in Dr. W. C. Rontgen. Glasser, O. Charles C Thomas, Springfield; 1945.
- Seliger, H.H. Wilhelm Conrad Röntgen and the Glimmer of Light. Physics Today, Nov.: 25-31; 1995.
- Steidley, K.D. The Radiation Phosphene. Vision Research 30: 1139-1143; 1990.
I would like to express my gratitude to Howard Seliger and David Steidley for their invaluable assistance.