Editing Joseph von Fraunhofer (section)

== Invention and scientific research ==
[[Image:Fraunhofer spectroscope.JPG|thumb|right|Fraunhofer demonstrating the spectroscope]]
One of the most difficult operations of practical optics during the time period of Fraunhofer's life was accurately [[polishing]] the spherical surfaces of large [[Objective (optics)|object glasses]]. Fraunhofer invented the machine which rendered the surface more accurately than conventional [[Grinding (abrasive cutting)|grinding]]. He also invented other grinding and polishing machines and introduced many improvements into the manufacture of the different kinds of glass used for optical instruments, which he always found to have flaws and irregularities of various sorts.<ref name=amer/>

In 1811, he constructed a new kind of [[Glass production|furnace]], and during his second melting session when he melted a large quantity of glass, he found that he could produce [[flint glass]], which, when taken from the bottom of a vessel containing roughly 224 pounds of glass, had the same [[refraction|refractive]] power as glass taken from the surface. He found that English [[crown glass (optics)|crown glass]] and German table glass both contained defects which tended to cause irregular refraction. In the thicker and larger glasses, there would be even more of such defects, so that in larger telescopes this kind of glass would not be fit for objective lenses. Fraunhofer accordingly made his own crown glass.<ref name=amer/>

It was thought that the accurate determination of power for a given medium to refract rays of light and separate the different colors which they contain was impeded by the absence of precise boundaries between the [[color]]s of the [[spectrum]], making it difficult to accurately measure the angle of refraction. To address this limitation, Fraunhofer performed a series of experiments for the purpose of producing [[monochromatic light|homogeneous light]] artificially, and unable to effect his object in a direct way, he did so by means of lamps and [[Prism (optics)|prism]]s.<ref name=amer/>

=== Discovery of dark absorption lines ===
[[File:DBP 1987 1313 Joseph von Fraunhofer, Sonnenspektrum.jpg|thumb|upright=1.2|Illustration of solar spectrum drawn and colored by Joseph von Fraunhofer with dark lines named after him (1987 [[Deutsche Bundespost|DBP]]'s stamp on 200th anniversary of birthday of Fraunhofer)]] 
By 1814, Fraunhofer had invented the modern [[spectroscope]].<ref name="Brand 37">{{cite book |title=Lines of Light: The Sources of Dispersive Spectroscopy, 1800–1930 |last=Brand |first=John C. D. |publisher=Gordon and Breach Publishers |year=1995 |isbn=978-2884491624 |pages=37–42}}</ref> In the course of his experiments, he discovered a bright fixed line which appears in the orange color of the spectrum when it is produced by the light of [[fire]]. This line enabled him afterward to determine the absolute power of refraction in different substances. Experiments to ascertain whether the solar spectrum contained the same bright line in orange as the line produced by the orange of fire light led him to the discovery of 574 dark fixed lines in the solar spectrum. Today, millions of such fixed absorption lines are now known.<ref name=amer/><ref>See:
* Joseph Fraunhofer (1814–1815) [https://books.google.com/books?id=2-AAAAAAYAAJ&pg=PA203 "Bestimmung des Brechungs- und des Farben-Zerstreuungs – Vermögens verschiedener Glasarten, in Bezug auf die Vervollkommnung achromatischer Fernröhre"] (Determination of the refractive and color-dispersing power of different types of glass, in relation to the improvement of achromatic telescopes), ''Denkschriften der Königlichen Akademie der Wissenschaften zu München'' (Memoirs of the Royal Academy of Sciences in Munich), '''5''': 193–226; see especially pages 202–205 and the plate following page 226.
* Reprinted, with additional findings and notes, in: Joseph Fraunhofer (1817) [http://gallica.bnf.fr/ark:/12148/bpt6k15065z/f276.image.r=Annalen%20der%20physik%20(Leipzig).langEN "Bestimmung des Brechungs- und des Farben-Zerstreuungs – Vermögens verschiedener Glasarten, in Bezug auf die Vervollkommnung achromatischer Fernröhre"] {{Webarchive|url=https://web.archive.org/web/20161010055445/http://gallica.bnf.fr/ark:/12148/bpt6k15065z/f276.image.r=Annalen%20der%20physik%20(Leipzig).langEN |date=10 October 2016 }} (Determination of the refractive and color-dispersing power of different types of glass, in relation to the improvement of achromatic telescopes), ''Annalen der Physik'', '''56''':  264–313; see especially pages 278–286.</ref>

Continuing to investigate, Fraunhofer detected dark lines also appearing in the spectra of several bright [[star]]s, but in slightly different arrangements. He ruled out the possibility that the lines were produced as the light passes through the [[Earth’s atmosphere]]. If that were the case they would not appear in different arrangements. He concluded that the lines originate in the nature of the stars and [[sun]] and carry information about the source of light, regardless of how far away that source is.<ref name="nautilus"/>  He found that the spectra of [[Sirius]] and other first-magnitude stars differed from the sun and from each other, thus founding [[Astronomical spectroscopy|stellar spectroscopy]].<ref>Fraunhofer (1814–1815), [https://books.google.com/books?id=2-AAAAAAYAAJ&pg=RA1-PA220 pages 220–221] {{Webarchive|url=https://web.archive.org/web/20240310212337/https://books.google.com/books?id=2-AAAAAAYAAJ&pg=RA1-PA220 |date=10 March 2024 }}:

''Original'': ''Ich habe auch mit derselben Vorrichtung Versuche mit dem Lichte einiger Fixsterne erster Grösse gemachte. Da aber das Licht dieser Sterne noch vielmal schwächer ist, als das der Venus, so ist natürlich auch die Helligkeit des Farbenbildes vielmal geringer. Demohngeachtet habe ich, ohne Täuschung, im Farbenbilde vom Lichte des Sirius drey breite Streifen gesehen, die mit jenen vom Sonnenlichte keine Aehnlichkeit zu haben scheinen; einer dieser Streifen ist im Grünen, und zwey im Blauen. Auch im Farbenbilde vom Lichte anderer Fixsterne erster Grösse erkennt man Streifen; doch scheinen diese Sterne, in Beziehung auf die Streifen, unter sich verschieden zu seyn.''

''Translation'': With the same device [i.e., spectroscope], I've also made some experiments on the light of some stars of the first magnitude.  Since the light of these stars is many times weaker than that of Venus, so naturally, the brightness of the spectrum is also many times less. Notwithstanding, I have seen – without any illusion – three broad stripes in the spectrum of Sirius, which seem to have no similarity to those of sunlight; one of these stripes is in the green, and two in the blue. Also, in the spectrum of the light of other fixed stars of the first magnitude, one detects stripes; yet these stars, in regard to the stripes, seem to differ among themselves.</ref>

These dark fixed lines were later shown to be mostly atomic absorption lines, as explained by [[Gustav Kirchhoff|Kirchhoff]] and [[Robert Bunsen|Bunsen]] in 1859,<ref>See:
* Gustav Kirchhoff (1859) [https://books.google.com/books?id=CMgAAAAAYAAJ&pg=PA662 "Ueber die Fraunhofer'schen Linien"] (On Fraunhofer's lines), ''Monatsbericht der Königlichen Preussische Akademie der Wissenschaften zu Berlin'' (Monthly Report of the Royal Prussian Academy of Sciences in Berlin), 662–665.
* Gustav Kirchhoff (1859) [https://books.google.com/books?id=uksDAAAAYAAJ&pg=RA1-PA251 "Ueber das Sonnenspektrum"] (On the sun's spectrum), ''Verhandlungen des naturhistorisch-medizinischen Vereins zu Heidelberg'' (Proceedings of the Natural History / Medical Association in Heidelberg), '''1''' (7): 251–255.</ref> with the rest identified as [[telluric contamination|telluric lines]] originating from absorption by [[oxygen]] molecules in the [[Earth's atmosphere]]. These lines are still called ''[[Fraunhofer lines]]'' in his honor; his discovery had gone far beyond the half-dozen apparent divisions in the solar spectrum that had previously been noted by [[William Hyde Wollaston|Wollaston]] in 1802.<ref>William Hyde Wollaston (1802) [http://rstl.royalsocietypublishing.org/content/92/365.full.pdf+html "A method of examining refractive and dispersive powers, by prismatic reflection,"] {{Webarchive|url=https://web.archive.org/web/20160319115418/http://rstl.royalsocietypublishing.org/content/92/365.full.pdf+html |date=19 March 2016 }} ''Philosophical Transactions of the Royal Society'', '''92''': 365–380; see especially p. 378.</ref>

=== Invention of optical instruments ===
Fraunhofer also developed a [[diffraction grating]] in 1821, after [[James Gregory (astronomer and mathematician)|James Gregory]] discovered the phenomenon of diffraction grating and after the American astronomer [[David Rittenhouse]] invented the first manmade diffraction grating in 1785.<ref>See:
* Frauhofer. Jos. (1821) [https://books.google.com/books?id=k-EAAAAAYAAJ&pg=RA1-PA3 "Neue Modifikation des Lichtes durch gegenseitige Einwirkung und Beugung der Strahlen, und Gesetze derselben"] (New modification of light by the mutual influence and the diffraction of [light] rays, and the laws thereof), ''Denkschriften der Königlichen Akademie der Wissenschaften zu München'' (Memoirs of the Royal Academy of Science in Munich), '''8''': 3–76.
* Fraunhofer, Jos. (1823) [http://gallica.bnf.fr/ark:/12148/bpt6k15083w/f353.image "Kurzer Bericht von den Resultaten neuerer Versuche über die Gesetze des Lichtes, und die Theorie derselben"] {{Webarchive|url=https://web.archive.org/web/20160216172921/http://gallica.bnf.fr/ark:/12148/bpt6k15083w/f353.image |date=16 February 2016 }} (Short account of the results of new experiments on the laws of light, and the theory thereof) ''Annalen der Physik'', '''74'''(8): 337–378.</ref><ref>{{cite journal |author=Parker AR |title=A geological history of reflecting optics |journal=Journal of the Royal Society, Interface |volume=2 |issue=2 |pages=1–17 |date=March 2005 |pmid=16849159 |pmc=1578258 |doi=10.1098/rsif.2004.0026}}</ref> Fraunhofer was the first who used a diffraction grating to obtain line spectra and the first who measured the wavelengths of spectral lines with a diffraction grating.

Ultimately, however, his primary passion was still practical optics; he once wrote that "In all my experiments I could, owing to lack of time, pay attention to only those matters which appeared to have a bearing upon practical optics".<ref>Prismatic and Diffraction Spectra: Memoirs (1899) Tr. & Ed. J. S. Ames p. 10</ref>