Optics & Photonics News - November 2009

Janssen was examining sunlight with a telescope and spectroscope on August 18,
1868, when he noticed an unknown bright yellow spectral line signature—it was the
first evidence of helium.

with a telescope and spectroscope on August 18, 1868, when he noticed an unknown bright yellow spectral line signature—it was the first evidence of helium. It appeared at a wavelength of 587.49 nm in the spectrum of the chromosphere of the sun. Janssen is also credited with demonstrating the gaseous nature of the red solar prominences that were observed during solar eclipses.

Janssen had studied mathematics and physics, but he was mainly involved in scientific expeditions to Asia, Africa, Europe, and the Pacific Islands from 1860 to 1905. On these trips, he studied eclipses, and, that year, he had discovered a technique to observe solar prominences that occurred in the absence of an eclipse.

Tests were soon carried out at the College of Chemistry in London to reproduce the line that Janssen had observed. Although its existence was confirmed, no one knew its source. Janssen postulated that it was an element that had not yet been discovered on our planet—a conjecture that attracted much ridicule since no elements had been detected on the sun that were not known to exist on earth.

Meanwhile, on October 20 of that same year, another scientist made the same independent discovery. In a remarkable leap of intuition, the British astronomer Joseph Norman Lockyer (1836-1920) combined an astronomical telescope, which he built himself, with a prism spectroscope. Using this setup, Lockyer discovered the same yellow line that Janssen had observed. He named it the D3 Fraunhofer line because it was near the known D1 and D2 lines of sodium. He also concluded that its origin was an unknown chemical element that had not yet been found on earth.

Lockyer collaborated with the English chemist Edward Frankland, who had previously investigated how the spectrum of elements related to their physical state, on the identification of helium. They named the element helium, after the Greek word for the sun (helios).

(Lockyer is also well-known for establishing the journal Nature in 1896. One of his primary objectives in doing so was to facilitate the communication of ideas between disparate scientific disciplines; this goal seems so relevant today. For the next 50 years, he remained the journal’s editor until just before his death in 1920.)

Some 25 years after Janssen and Lockyer discovered helium on the sun, a Scottish chemist found the first evidence of it on our home planet. In 1895, William Ramsay discovered helium in uranium minerals. In another example of independent discovery, two Swedish chemists—Per Teodor Cleve and Nils Abraham Langlet—also discovered helium in a mineral called cleveite at around the same time.

Concluding thoughts

In 1835, the French positivist philosopher Auguste Comte wrote that people would never understand the chemical constitution of the stars. Thankfully, through spectroscopy, science has proven Comte wrong. In 1904, after Pierre Janssen and J. Norman Lockyer independently discovered helium on the sun, Lockyer wrote at the beginning of his book Studies In Spectrum Analysis: “The work of the true man [woman] of Science is a perpetual striving after a better and closer knowledge of the planet on which his lot is cast, and of the universe in the vastness of which that planet is lost.” t

Barry R. Masters ( brmail2001@yahoo.com), OSA Fellow, SPIE Fellow, is with the department of biological engineering, Massachusetts

Institute of Technology, Cambridge, Mass., U.S.A. Member

[ References and Resources ]

>> R. Bunsen. Gesammelte Abhandlungen. 3 Bände, Leipzig, Engelmann (1904).

>> G. Kirchhoff. “Über das Verhältnis zwischen dem Emissionsver-mögen und dem Absorptionsvermögen der Körper für Wärme und Licht.” Annalen der Physik 109, 275-301 (1860).

>> J.N. Lockyer. The Spectroscope and Its Applications. London, MacMillan and Co. (1873). [available on Google Books]

>> J.N. Lockyer. Studies in Spectrum Analysis. London: Kegan Paul, Trench, Trübner & Co. Ltd. (1904).

>> Sir I. Newton. Opticks, Fourth Edition, 1730 [first published 1704], New York, Dover Publications (1952).

>> J. Agassi. The Kirchhoff-Planck Radiation Law, Science 156, 30-7 (1967).

>> W. McGucken. Ninteenth-Century Spectroscopy, Development of the Understanding of Spectra 1802-1897. Baltimore, Johns Hopkins Press (1969).

>> A.J. Meadows. Science and Controversy: A Biography of Sir Norman Lockyer. Cambridge, Mass., MIT Press (1972).

>> H. Kangro. Gustav Kirchhoff: Untersuchungen über das Sonnen-spectrum und die Spectren der chemischen Elemente und weitere ergänzebde Arbeiten aus den Jahren 1859-1862. Osanabrück, Zeller (1972).

>> J.A. Bennett. The Celebrated Phaenomena of Colours: The Early History of the Spectroscope. Cambridge, U.K., Whipple Museum of the History of Science (1984).