ScienceWeek

HISTORY OF PHYSICS: ASTROSPECTROSCOPY

The following points are made by Barbara J. Becker (Science 2003 301:1332):

1) In October 1859, German physicist Gustav Kirchhoff (1824-1887) announced the results of his investigations with chemist Robert Bunsen (1811-99) on the dark lines that interrupt the otherwise continuous solar spectrum (1). These lines had puzzled solar observers and theorists alike since they were first seen in 1814 by German optician Josef von Fraunhofer (1787-1826) (2).

2) Now it seemed that Bunsen and Kirchhoff had finally confirmed what others had long suspected, namely, that an individual metal produces its own characteristic pattern of bright spectral lines when it is burned. Furthermore, Kirchhoff asserted that Fraunhofer's lines "exist in consequence of the presence, in the incandescent atmosphere of the Sun, of those substances which in the spectrum of a flame produce bright lines at the same place." News of his claim spread quickly throughout the scientific world, and many astronomers quickly realized that observations of spectral lines might lead to information concerning the chemical composition of the stars.

3) This was in defiance of an edict of the French philosopher Auguste Comte (1798-1857), who in 1835 had stated: "We can imagine the possibility of determining the shapes of stars, their distances, their sizes, and their movements, but there is no means by which we will ever be able to examine their chemical composition" (5).

4) Despite Comte, coupling the spectroscope to the astronomical telescope in the 19th century revolutionized the way astronomy was performed, realigning the very boundaries of what astronomers considered to be acceptable research. A quarter of a century after Kirchhoff's announcement, the noted historian of astronomy Agnes Clerke (1842-1907) marveled at the youthful audacity of a new science she called "astronomical or cosmical physics". "It promises everything," she wrote, "it has already performed much; it will doubtless perform much more."(3,4)

References (abridged):

1. G. Kirchhoff, Philos. Mag., 4th ser., 21, 196 (1860).

2. Published in three segments. J. Fraunhofer, Edinburgh J. Sci. 7, 101-113 and 251-252 (1827); 8, 7-10 (1828).

3. H. E. Roscoe to G. G. Stokes, 24 February 1860, from Stokes papers, Add. MS 7656.R788, Cambridge University Library.

4. W. De la Rue, Chem. News 4, 130 (1861).

5. A. Comte, Cours de Philosophie Positive, vol. 2 (Baillière, Paris, ed. 2, 1864) p. 6, translation by the author.

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ON GRATINGS, SPECTROSCOPY, AND HENRY ROWLAND

The precision of spectroscopy depends ultimately upon the precision of some human being's hand and eye. In a research spectroscope, a diffraction grating rather than the familiar prism disperses the radiant energy of light. This is a flat or spherically curved slab of mirrored glass from which the light is diffracted by parallel lines -- hence the term grating --scratched on the reflecting surface. The resolution of a grating, the narrowness and faintness of the spectral lines it captures, is a function of the number of lines per inch and the exactitude with which they are spaced in the scratching.

Until the advent of electronic feedback control technology following the Second World War, the perfection of spectroscopy was owing to the pleasure that Henry Augustus Rowland (1848-1901) found in contending with physical reality, after the manner of Galileo, with bare hands. His tangible legacy is 1000 of the most perfect diffraction gratings and a million or more precisely established spectral lines in the catalogue recorded from them. The founding professor of physics, at age 27 in 1875, at Johns Hopkins University, Rowland acted on his determination to get the foundation work of spectroscopy done quickly and precisely. He settled on a standard of 14,400 two-inch lines of identical width and depth, to be plowed in the mirrored surface exactly parallel to one another across each inch and at exactly the same spacing from one another across a width of five or six inches.

To eliminate the focusing lens and the loss of light it would entail, he hit on the self-focusing spherical surface. Rowland once boasted that he never saw a machine that he did not immediately understand. He recognized that to make the perfect "ruling engine" to rule the perfect grating required the perfection of the critical engine part: the lead screw that would advance a diamond stylus exactly 1.44 x 10^(-4) inches at each step across a stretch of five or six inches. At 20 threads to the inch, each full rotation of the screw would advance the diamond stylus of his ruling engine one-twentieth of an inch; each of the 720 settings in each rotation advanced it 1.44 x 10^(-4) inches. To grind the perfect thread in the 9-inch lead screw, Rowland invented a new machine tool. This was an 11-inch-long grinding or lapping nut with the same threading, which he split lengthwise in four sections. With the segments reassembled around the screw, he could adjust the tightness of the nut through the course of grinding with emery powder and oil and finally the rouge used to polish telescope mirrors. "Now grind the screw in the nut," Rowland wrote, "making the nut pass backwards and forwards over the whole screw... Turn the nut end for end every ten minutes and continue for two weeks." In that time the high spots on the threading of the nut and the screw found one another and wore themselves away.

Adapted from: Gerard Piel: The Age of Science: What Scientists Learned in the 20th Century (Basic Books, New York 2001, p.54) More information at: http://www.amazon.com/exec/obidos/ASIN/0465057551/scienceweek

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