Paschen, Louis Carl Heinrich Friedrich
PASCHEN, LOUIS CARL HEINRICH FRIEDRICH
(b. Schwerin, Mecklenburg, 22 January 1865; d. Potsdam, Germany, 25 February 1947)
Friedrich Paschen, “probably the greatest experimental spectroscopist of his time,”1 was born into a Lutheran family of scientifically inclined Mecklenburg officers, military and civil. His paternal grandfather, H. C. Friedrich Paschen (1804–1873), was director of the Mecklenburg geodetic survey and a noted astronomer. An uncle, Carl Paschen (1835–1911), who rose to the rank of admiral in the German navy, was a well-known hydrographer. Paschen himself, although he incorporated some official virtues—and although, as was customary, he became a lieutenant in the reserve2—seems nonetheless to have reacted against the authoritarian structure and the social-political attitudes of the German officer class. Apparently against the wishes of his family, and with no independent income, he resolved to accept the greater hazards of an academic career.3
After completing his secondary education in Schwerin, Paschen began his university studies in 1884 at Strasbourg, where he joined the group of disciples around August Kundt, the most charismatic and influential professor of physics in Germany, who happened also to be a native of Mecklenburg and a graduate of the Schwerin Gymnasium. After two years at Strasbourg, Paschen studied for one year at the University of Berlin, then returned to Kundt for supervision of his doctoral research. The degree was conferred in September 1888.4 Paschen’s doctoral research, suggested to him by Kundt, established “Paschen’s law”: that the sparking voltage depends only on the product of the gas pressure and the distance between the electrodes—one of the first and most important of the numerous scaling laws in this field.5
From October 1888 to April 1891 Paschen was Wilhelm Hittorf’s last assistant in the Physical Institute of the Catholic Academy (subsequently University) of Münster. In keeping with Hittorf’s interests and the experimental facilities available in consequence of them, Paschen turned to electrolytic solutions and, giving the first proofs of his enormous energy, in two years published seven papers on a variety of investigations of electrolytic potentials. It was here, under Hittorf, that Paschen learned the value and technique of precision measurements.6 Hittorf retired in 1890, and at Easter 1891 Paschen became Heinrich Kayser’s teaching assistant in the Physical Institute of the Technische Hochschule at Hannover. Paschen continued in this position until Easter 1901, serving under Conrad Dieterici after Kayser’s departure for Bonn in the summer of 1894.
Paschen had qualified as lecturer (habilitiert) at Hannover in the spring of 1893; and in March 1895, in return for his refusal of a position at Aachen, the Prussian government created a permanent lectureship (etatsmässige Dozentur) in physics and photography for him. This post gave him a relatively comfortable income of 3,300 marks in 1895, rising to about 4,000 marks by 1900, and a relatively large amount of time for research.7
In the late 1880’s Kirchhoff’s function (the universal but unknown temperature and wavelength dependence of the ratio of the radiant energy emitted to that absorbed by a body in thermal equilibrium—and, consequently also the dependence of the radiant energy in a cavity in thermal equilibrium upon those two variables) was beginning to draw considerable attention from theorists following the bolometric measurements of S. P. Langley and others.8 Kayser proposed that Paschen improve upon Langley’s results by more extensive measurements using the reflection gratings with which the institute had been well supplied for Kayser’s and Carl Runge’s spectroscopic work.9 Gratings should have the advantage that their spectra, in contrast to prismatic spectra, may be chosen “normal” —that is, to disperse equal wavelength intervals into equal angular intervals—so that intensity measurements will give the desired distribution function directly. Only after a year spent building up the necessary apparatus—including the most sensitive galvanometer (of the Thomson astatic type) constructed until then or for years afterward—did Paschen discover late in 1892 that the gratings were unusable because the metal on which they were ruled showed irregular selective reflection in the infrared.10
But now committed to this problem, which would absorb most of his research efforts for the ten years at Hannover, Paschen turned to prismatic spectra and made a very accurate investigation of the dispersion of fluorite (correcting Rubens’ measurements). At the same time he determined the infrared absorption by carbon dioxide and water vapor (obtaining results which, although presumed to have been superseded by Rubens, remained in 1913 the strongest evidence in favor of Bjerrum’s quantum theory of molecular absorption).11 Paschen also investigated the much mooted question of whether heat alone could bring gases to radiate, demonstrating—in contract with the results of Ernst Pringsheim—the existence of infrared spectral lines produced by merely heating the gas.12
Paschen spent much of 1894 observing the deflections of his galvanometer attached to a very delicate bolometer irradiated by heated platinum with various surfacings; but the reduction of this data to emissive power as a function of λ and T, espically the transformation from a prismatic to a normal spectrum, required extremely laborious computations. By the summer of 1894 he had strong indications that λmax · T =constant, “or: the frequency of the main thermal vibrations of the molecular parts of an absolutely black body is proportional to the absolute temperature.”13 Paschen, who was never willing to take time from the laboratory to find out in the library what others had done—nor, indeed, ever to write a review article in any of the fields in which he was to become an authority—seems to have been ignorant of W. Wien’s publication of thermodynamic deductions of this “displacement law” in 1893 and 1894. Paschen was, however, aware of the general growth of interest and activity in this field, especially in Berlin (Rubens, Pringsheim, Lummer, among others); and in the summer of 1895, in view of the growing competition, Paschen published his result.14
But to complete the computation of all those “hundreds of curves” from which Paschen hoped to induce Kirchhoff’s function would take many months more. It was thus not difficult for Carl Runge, excited by Ramsay’s announcement in March 1895 of the discovery of terrestrial helium and bereft of his experimental collaborator Kayser, to persuade Paschen that the investigation of the spectrum of this new element was far more important to science. Compared with Paschen’s infrared researches, this optical spectroscopy proved to be child’s play: in one day he obtained for Runge the yellow D3 helium line, and within three months they brought out two papers giving an astonishingly accurate inventory of the helium lines and an astonishingly successful arrangement of them into series.15 Overnight Paschen acquired an international reputation; and accompanying Runge to England in September 1895, he was received most warmly by British physicists. 16
In the fall of 1895 Paschen returned to the calculation of his emissive power curves, and by the spring of 1896 he had found I = c1λ-aexp(-c2/λT) with α = 5.5, for an iron oxide surface.17 The formulas for this curve previously proposed by V. A. Michelson and H. F. Weber, involving exponential dependence, had undoubtedly been suggestive; and Runge’s suggestion that the energy curves be plotted on logarithmic scales was also helpful.18 Wien, who had evidently gotten into contact with Paschen after the publication of Paschen’s preliminary announcement in the summer of 1895, was informed in advance of these results by mail. Wien replied that he had deduced exactly this formula, but with α = 5, some time earlier. The derivation of what soon became known as Wien’s law—doubtless previously withheld from publication because of its highly arbitrary character—now appeared immediately following Paschen’s paper in the Annalen.19
In the laboratory, however, Paschen had been obliged by lack of space and funds to forgo entirely his bolometric work from mid-1895 to the end of 1896. Instead he joined Runge in the quest for spectral series, first unsuccessfully with argon and then successfully with the homologous oxygen-sulfur-scandiumtellurium spectra. Through this collaboration Paschen became fully familiar with the field that would occupy him almost exclusively after the question of the blackbody radiation formula had finally been settled. That question certainly could not be settled by measurements on heated surfaces; to assert definitely that his (and Wien’s) formula was the true one, and to fix the value of α, would require a far more perfect realization of ideal blackbody radiation.
For this purpose Paschen’s competitors, enjoying the ample resources of the Physikalisch-Technische Reichsanstalt at Berlin, had in 1895 begun the construction of elaborate “thermostatic cavities.” In the spring of 1897 Paschen, with a grant from the Berlin Academy and with the enlargement of the Hannover Institute, began to construct similar equipment.20 By the summer of 1898 he had found α = 5 and was “convinced that there can remain no further doubt about the correctness of the formula itself and of its constants.”21 Indeed, he was so convinced that when deviations between theory and experiment began to appear in 1899, he took them as indicative of undetected sources of experimental error. Only in 1900, when the Berlin experimentalists, working with much larger values of λ · T, found clear deviations from Wien’s law, did Paschen (like Planck) reconceptualize his experiments as a search for the limits of the law’s range of validity. Paschen construed the results positively as strong evidence for the validity of Planck’s formula in the intermediate region between Wien’s and Rayleigh’s formulas.22 In striking and curious contrast with the Berlin experimentalists, who were literally enraged at him, throughout his work on the blackbody radiation problem Paschen the pure experimentalist showed himself to be more than ready to enlist experiment in the service of theory. This continued to be characteristic of the man, of his work, and of some of his greatest successes.
In these years, despite the volume and importance of his work, it looked very much as if Paschen, like Runge, would remain stuck at Hannover.23 Finally, however, in February 1901, the University of Tübingen, after failing successively to secure Paul Drude, Philipp Lenard, and Hermann Ebert, appointed Paschen professor of physics at a salary of 3,500 marks, including living quarters in a moderately good institute.24 Here Paschen brought a wife in September 1901; here he raised a daughter and married her to his student Hermann Schüler in August 1920;25 here he kept a guest room “always standing ready for the physicists.” In 1908 the institute was substantially enlarged to accommodate the growing number of advanced students, including eventually many from abroad.26 In July 1915 Paschen refused a call to Göttingen as Eduard Riecke’s successor, gaining in return 9,000 marks for his institute and 3,000 marks per annum salary supplement.27 In 1919 he was offered and actually accepted the Bonn chair of physics in succession to Kayser; but he reneged early in 1920 because he was persuaded that, in consequence of the political and above all the economic situation, the move would be to the disadvantage of his research work.28
Paschen had lost a year or two of research in transferring to Tübingen, and the diverse resources of the institute there led to forays into other fields— radioactivity and the nature of X rays, canal rays, and the mechanism of light emission. By 1908, however, he was focusing again on the problem of spectral series. Important forces bringing his attention back to this problem, to which he would devote himself exclusively for the remainder of his career, were, on the one hand, the presence of Walther Ritz at Tübingen during the winter of 1907–1908 and, on the other hand, Arno Bergmann’s discovery in 1907 of new series in the infrared spectra of the alkalies (the so-called f series). Paschen now had, as he had not had at Hannover in the early 1890’s, the facilities necessary for a systematic bolometric search for infrared spectral lines, above all the large-capacity high-voltage storage batteries to maintain intense, steady discharges. Returning to helium, in the spectrum of which he had previously detected bolometrically (June 1895) a few lines predicted by Runge’s series formulas, he found in the spring of 1908 additional lines that did not fit in that series system. Paschen was looking everywhere for the impurity responsible for these lines when a letter arrived from Ritz announcing his newly invented combination principle and suggesting that helium lines might exist at precisely those wavelengths Paschen had observed. Following this striking confirmation, Ritz suggested that Paschen look for hydrogen lines at frequencies ν = N (1/32–1/m2), m = 4, 5 …, and this “Paschen series” was soon found.29
In 1899 Thomas Preston had presented evidence that the magnetic splitting of spectral lines (Zeeman effect) was characteristic for the series to which they belonged, and in 1900 Runge and Paschen had begun a very careful investigation of Preston’s rule. Paschen had been able to do no more than make the requisite photographs before he left for Tübingen; the striking quantitative results published under their names were entirely Runge’s.30 By 1905, however, Paschen had begun to equip himself to continue this work; and from 1907 he and his students employed the Zeeman effect extensively and with great success as an aid in identifying series lines.
At the same time, however, they found a large number of apparent exceptions to Preston’s rule. In the simplest case those were very narrow doublet or triplet line groups showing the “normal” splitting pattern characteristic of a single line rather than the anticipated superposition of the “anomalous” splittings of the individual components of the group. Paschen, investigating this general circumstance with his student Ernst Back, and basing himself upon Ritz’s conception of a spectral line as the combination of two independently subsisting terms, showed in 1912 that in sufficiently strong magnetic fields—i.e.; fields strong enough for the magnetic splitting to be large compared with the separation of the components of the line group—all the splitting patterns transform themselves into the “normal” pattern. This “PaschenBack effect” was immediately seized upon as potentially one of the most revealing clues to atomic structure and the mechanism of emission of spectral lines.31
In the last days of July 1914 the intense activity at Paschen’s institute ceased abruptly. The German students and the institute staff rushed to the colors, and the foreign students fled over the Swiss border; Paschen himself seems not to have had the least inclination to participate in the war effort.32 In the summer of 1915, with the aid of a single technician recalled from the army, Paschen again took up the most interesting problem upon which he had been working in 1914— “Bohr’s helium lines,” the lines previously construed as the sharp series of hydrogen but now ascribed by Bohr to ionized helium. In the course of this work, which was initially intended to check Bohr’s prediction of a small difference between the Rydberg constant, N, for hydrogen and helium, and which was hampered by the diffuseness of the lines, Paschen discovered that a particular layer in the negative glow inside the common cylindrical-cathode Geissler tube gave especially sharp and complete spectra. Following up this observation, he developed the Paschen hollow-cathode discharge tube, in which, under the right conditions, the glow discharge retreats entirely into the largely field-free interior of a boxlike cathode. This device, which was the basis of much of the subsequent work on series and multiplet structure by Paschen and his students, showed the fine structure of Bohr’s helium lines with extraordinary clarity and completeness.33 (Here the series structure is the fine structure.)
Paschen had already reached this point when, late in 1915, Arnold Sommerfeld wrote inquiring about data with which to compare the relativistic fine structure demanded by the extension of Bohr’s theory that he was then developing. Paschen was impressed; enlisting himself in the service of Sommerfeld’s theory, he spent all his free time in the following six months confirming its predictions in detail, so far as possible.34 The single paper presenting his results was immediately recognized as the tremendous advance in knowledge that it was.35
After the war spectroscopic activity in Paschen’s institute increased rapidly. During the six years before his departure in 1924 Tübingen was unquestionably the most important center of atomic spectroscopy in Germany, at a time when this technique was far and away the most important for the advance of theoretical atomic physics. Paschen’s own outstanding successes were the ordering of the neon spectrum—almost 1,000 lines—into spectral series; the evocation of the missing combinations between complex spectral terms by magnetic fields of appropriate strength (violation of the selection rule for the total angular momentum quantum number); and the first analysis of the spectra of an atom in its doubly ionized, as well as its neutral, and singly ionized states.36 At the same time his associates Ernst Back and Alfred Landé were in the forefront of research on multiplet structure and Zeeman effects.
Paschen accepted, as few other experimentalists did, the priorities and guidance of atomic theorists. In 1922, persuaded that Lané could do more for him in this respect than any other available young theorist, he fought recklessly, tenaciously, and ultimately successfully against the social-political prejudices of his university for Landé’s appointment to Tübingen’s professorship (Extraordinariat) for theoretical physics.37
In July 1924 Paschen was looking forward to spending that fall and winter in the United States as the second German physicist (after Sommerfeld) to receive the compliment of a visiting professorship there since the war. But September found Paschen in Berlin, not Ann Arbor, for the month before he had been offered the presidency of the Physikalisch Technische Reichsanstalt, as successor to Nernst.38 Paschen accepted the post at the urging of the leading Berlin physicists, and with the intent of restoring basic research as a principal function of the institution. In this endeavor he had only very limited success because of budgetary constraints, bureaucratic resistance, and Paschen’s own limitations as administrator and politician. After scarcely a year in the post he was letting it be known that he would be glad to return to a university chair.39 He stayed on, however, gradually building up a spectroscopic laboratory to continue his previous line of research. As honorary professor, Paschen lectured at the University of Berlin on some topic in spectroscopy or physical optics for two hours a week every term. As a member of the Berlin Academy (from July 1925) and of various committees and commissions, and as president of the Deutsche Physikalische Gesellschaft (1925–1927), he played a prominent role—although not a key role—in the lively scientific life in Berlin during the later Weimar period.
Paschen’s post, the highest to which a German experimental physicist could aspire, was ipso facto coveted by Johannes Stark; immediately following the Nazi seizure of power, Stark had himself appointed to it, effective 1 May 1933.40 Forced out of his office and into retirement, Paschen was still able to continue working for a few years in his laboratory—at the cost of considerable difficulty and personal humiliation. Finally he withdrew to his home in Charlottenburg, where he confined himself to the evaluation of his spectrographs.41 In November 1943 his home and all his possessions went up in flames in a bombing raid. Paschen then moved to Potsdam, where, weakened by postwar deprivations, he died of pneumonia early in 1947.
1. S. Tolansky, “Friedrich Paschen,” p. 1040.
2. Paschen, “Vita,” Dissertation (1888); University of Tübingen Archives, 128/Paschen.
3. W. Gerlach, “Friedrich Paschen,” p. 277; Paschen to Kayser, 14 Nov. 1895.
4. Paschen, “Vita,” Dissertation (1888); “Antrittsrede,” in Sitzungsberichte der Deutschen Akademie der Wissenschaften zu Berlin (1925), cii.
5. Paschen, “Ueber die zum Funkenübergang…erforderliche Potentialdifferenz,” in Annalen der Physik und Chemie, n.s. 37 (1889), 69–96; J. J. Thomson, The Conduction of Electricity Through Gases, 2nd ed. (Cambridge, 1906), pp. 451 ff.
6. Paschen, “Antrittsrede” (1925), p. cii.
7. Paschen to Kayser, 17 Mar. 1895; 18 July 1895; 14 Nov. 1895; 8 Feb. 1898.
8. H. Kangro, Vorgeschichte des Planckschen Strahlungsgesetzes.
10. Kangro, Vorgeschichte …, pp. 60–73.
11. Kangro, “Ultrarotstrahlung …,” p. 181.
12. Kangro, Vorgeschichte …, loc. cit.
13. Paschen, “Über Gesetzmässigkeiten in den Spectren fester Körper und über eine neue Bestimmung der Sonnentemperatur,” in Nachrichten der Gesellschaft der Wissenschaften zu Göttingen, Math.-phys. Kl. (1895), 294–304, dated June 1895.
15. “Spielerei”: Paschen to Kayser, 18 July 1895; Iris Runge, Carl Runge, pp. 73–74. Henry Crew, visiting Hannover at this time, found Paschen reminded him of J. E. Keeler, while his “laboratory like Nernst’s in apparent dissorder— using concave grating without any mounting, simply sets up the three parts in a room.” Crew, “Diary,” 12 July 1895 (American Institute of Physics, New York).
16. Paschen to Kayser, 14 Nov. 1895; Runge, op. cit., p. 76.
17. Paschen, “Ueber Gesetzmässigkeiten in den Spectren fester Körper. Erste Mittheilung,” in Annalen der Physik, 3rd ser., 58 (1896), 455–492, dated May 1896; Kangro, Vorgeschichte …, pp. 74–89.
18. Paschen, “Ueber Gesetzmässigkeiten in den Spectren fester Körper. Zweite Mittheilung,” in Annalen der Physik, 60 (1897), 663–723, dated Jan. 1897, see 723.
19. Paschen to Kayser, 4 June ; H. Kangro, “Das Paschen-Wiensche Strahlungsgesetz.”
20. Paschen to Kayser, 2 Aug. 1896; Sitzungsberichte der Preussischen Akademie der Wissenschaften (8 Apr. 1897), 453; and (18 May 1899), 438.
21. Paschen to Kayser, 17 July 1898. This result was not published, however, until almost one year later in Sitzungsberichte der Akademie der Wissenschaften zu Berlin (1899), 405–420.
22. Kangro, Vorgeschichte …, pp. 165–179, 233.
23. Paschen to Kayser, 19 Jan. 1901
24. University of Tübingen Archives, 128/Paschen; Paschen to L. Graetz, 22 July 1901 (Deutsches Museum, Munich); Paschen to Kayser, 18 Feb. 1901. “Der Neubau des physikalischen Instituts füdie kgl. württemb. LandesUniversität Tübingen,” in Deutsche Bauzeitung, 24 (1890), 213, 217.
25. Paschen to Sommerfeld, 25.8.20 (SHQP mf33); Paschen to E. Wiedemann, 10.6.13 (Darmst).
26. Württemberg, Landtag, Kammer der Abgeordneten, “Begründung einer Exigenz von 125000 Mk. Zur Erweiterung des physikalischen Instituts der Universität Tübingen,” in Verhandlungen, 37. Landtag (1907), Beilagenband 1, Heft 15, pp. 16–18. H. M. Randall, who spent the year 1910–1911 at Tübingen, recalled that “Paschen offered to show me how each element of his entire infrared setup was constructed … By 1914 a complete infrared installation of the Paschen type had been set up at Michigan…” “Infrared Spectroscopy at the University of Michigan,” in Journal of the Optical Society of America,44 (1954), 97–103, Many examples could be given of the imitation of Paschen’s installations by former students.
27. University of Tübingen Archives, 128/Paschen and 117/904; Akten der Naturwissenschaftlichen Fakultät, Tübingen.
28. Paschen to Kayser, 18 June 1919; Paschen to Sommerfeld 25 Jan. 1919 [sic; actually 1920], Mar. 1920 (SHQP mf 33).
29. Paschen, “Zur Kenntnis ultraroter Linienspektra. I. (Normalwellenlängen bis 27000 Å.-E),” in Annalen der Physik,27 (1908), 537–570, received 12 Aug. 1908; W. Ritz, Gesammelte Werke, Pierre Weiss, ed. (Paris, 1911), 521–525.
30. Runge, op. cit., p. 108.
31. Paschen and E. Back, “Normale und anomale Zeemaneffekte,” in Annalen der Physik,39 (1912), 897–932; Paul Forman, “Back,” in DSB, I, 370–371; J. B. Spencer, Zeeman Effect, 1896–1913.
32. Paschen to Kayser, 4 Feb. 1916.
33. H. Schüler, “Erinnerungen eines Spektrokopikers …,” in H. Leussink et al., Studium Berolinense (Berlin, 1960), 816–826.
35. Paschen, “Bohr’s Heliumlinien,” in Annalen der Physik, 4th ser., 50 (1916), 901–940, received 1 July 1916.
36. Paschen, “Das Spektrum des Neon,” ibid.,60 (1919), 405–453, and “Nachtrag,” ibid.,63 (1920), 201–220; Paschen and E. Back, “Liniengruppen magnetisch vervollständigt,” in Physica (Eindhoven), 1 (1921), 261–273; and Paschen, “Die Funkenspektren des Aluminiums,” in Annalen der Physik, 4th ser., 71 (1923), 142–161, 537–571.
39. Paschen to Sommerfeld, 14 Dec. 1924 (Archive for History of Quantum Physics); W. Wien to Ministerialrat [?], 14 Jan. 1926 (University of Munich Archives, Personalakten W. Wien, EII-698); H. Schüler, in Physikalische Blätter, 3 (1947), 232–233.
40. Armin Hermann, “Albert Einstein und Johannes Stark,” in Sudhoffs Archiv …, 50 (1966), 267–286, sec 283, which includes material on Paschen’s role in the consideration of Stark for membership in the Berlin Academy, Dec. 1933-Jan. 1934.
41. Gerlach, op. cit., p. 279.
I. Original Works. The only lists of Paschen’s publications are the Royal Society Catalogue of Scientific Papers, XVII, 721–722; and Poggendorff, IV, 1121, 1286–1287 (under Runge); V, 618 (under Kayser), 946, 1078–1079 (under Runge), 1349 (under Weinland); VI, 1956–1957, 2291 (under R. A. Sawyer); VIIa, pt. 3, 506–507. The following additional items have come to my attention: Ueber die zum Funkenübergang in Luft, Wasserstoff und Kohlensäure bei verschiedenen Drucken erforderliche Potentialdifferenz, his doctoral dissertation at Strasbourg (Leipzig, 1889), differs significantly from the version in Annalen der Physik,37 (1889), 69–96; “Terrestrial Helium,” in Nature,52 (6 June 1895), 128, Chemical News …, 71 (14 June 1895), 286; and Chemikerzeitung,19 (1895), 977, written with C. Runge; “Ueber das Strahlungsgesetz des schwarzen Körpers,” in Annalen der Physik, 4th ser., 4 (1901), 277–298; “Eine neue Bestimmung der Dispersion des Flusspates im Ultrarot,” ibid., 299–303; “Bestimmung des selectiven Reflexionsvermögens einer Planspiegel,” ibid., 304–306; “Erweiterung des Seriengesetzes der Linienspectra auf Grund genauer Wellanlängenmessungen im Ultraroth,” in Comptes rendus du Congrès international de radiologie et électricité, Brussels 1910, I (Brussels, 1911), 588–600, also in Jahrbuch der Radioaktivität und Elektronik, 8 (1911), 174–186; and “Antrittsrede,” in Sitzungsberichte der Preussischen Akademie der Wissenschaften (2 July 1925), cii-civ.
Paschen’s MSS apparently were destroyed with his home in 1943. Some 4 letters from Paschen to N. Bohr, 4 to W. Gerlach, 1 to S. A. Goudsmit, 1 to L. Graetz, 37 to H. Kayser, 1 to J. Königsberger, 14 to A. Landé, 1 to A. G. Shenstone, 87 to A. Sommerfeld, 2 to J. R. Swinne, and 1 to E. Wiedemann are listed or cited in T. S. Kuhn et al., Sources for History of Quantum Physics (Philadelphia, 1967), 72–73. The items in the Darmstadter Collection, Flc(4) 1893, cited there, particularly the important collection of 37 letters to Kayser—17 Mar. 1895; 24 Mar. (1895); 18 July (1895), 1 Aug. 1895; 14 Nov. 1895; 25 Nov. 1895; 30 Dec. 1895; 7 Jan. 1896; 13 Feb. 1896; 6 May 1896; 4 June (1896); 19 July 1896; 25 July 1896; 2 Aug. 1896; 8 Feb. 1898; 23 Feb. 1898; 17 July 1898; 30 Dec. 1900; 3 Jan. 1901; 9 Jan. 1900 [sic; actually 1901]; 19 Jan. 1901; 22 Jan. 1901; 18 Feb. 1901; 7 June 1902; 5 July 1903; 7 July 1903; 9 June 1905; 16 June 1905; 19 Nov. 1910; 3 Oct. 1912; 14 Oct. 1913; 4 Feb. 1916; 2 July 1915 [sic; actually 1916]; 29 Mar. 1919; 18 June 1919; 11 Oct. 1921; 14 Sept. 1923—are now in the Staatsbibliothek Preussischer Kulturbesitz, Berlin-Dahlem. The Nachlass Stark in the same depository includes, 25 letters from Paschen to Stark: 11 Jan. 1905; 28 Jan. 1905; 6 Mar. 1905; 19 May (1906); 10 July 1906; 2 Aug. 1906; 29 Sept. 1906; 3 Oct. 1906; 10 Nov. (1906); 12 Feb. 1907; 3 June 1907; 19 June (1907); 21 June (1907); 27 June (1907); 29 Feb. 1908; 18 Mar. 1911; 15 Oct. 1911; 18 Oct. 1911; 20 Oct. 1911; 22 Oct. 1911; 3 Oct. 1918; 20 Oct. 1918; 4 May 1927; 6 July 1927; 2 Oct. 1927. There are 3 additional letters to N. Bohr—11 Jan. 1924; 30 Mar. 1924; Copenhagen; and at least 3 letters to W.F. Meggers —6 Sept. 1921; 14 June 1924; 15 Oct. 1925—in the Meggers Papers, American Institute of Physics, New York.
II. Secondary Literature. Paschen’s work on Kirchhoff’s emission function is discussed in detail by Hans Kangro, Vorgeschichte des Planckschen Strahlungsgesetzes. Messungen und Theorien der spektralen Energieverteilung … (Wiesbaden, 1970), summarized in Kangro’s “Das Paschen-Wiensche Strahlungsgesetz und seine Abanderung durch Max Planck,” in Physikalische Blatter, 25 (1969), 216–220, and touched upon in his “Ultrarotstrahlung bis zur Grenze elektrisch erzeugter Wellen: Das Lebenswerk von Heinrich Rubens,” in Annals of Science, 26 (1970), 235–259, and 27 (1971), 165–200. Paschen’s collaboration and personal relations with Carl Runge are described in Iris Runge, Carl Runge und sein wissenschaftliches Werk (Gottingen, 1949). The magneto-optical work of Paschen and his school up through the discovery of the Paschen-Back effect is discussed in detail in James Brooks Spencer, An Historical Investigation of the Zeeman Effect, 1896–1913, Ph.D. diss., U. of Wisconsin, 1964 (Ann Arbor, 1964). Some of their later work is discussed by P. Forman, “Alfred Lande and the Anomalous Zeeman Effect, 1919–1921,” in Historical Studies in the Physical Sciences, 2 (1970), 153–262.
There are no biographical studies of Paschen apart from the very few and spare obituary notices. The best of these is Walther Gerlach, “Friedrich Paschen,” in Jahrbuch der Bayerischen Akademie der Wissenschaften (1944–1948), 277–280–Paschen had been elected a corresponding member in 1922. Others are S. Tolansky, “Friedrich Paschen,” in Proceedings of the Physical Society of London, 59 (1947), 1040–1041; H. Schuler, “Friedrich Paschen,” in Physikalische Blatter, 3 (1947), 232–233; R. Seeliger, “Nachruf auf Friedrich Paschen,” in Jahrbuch der Deutschen Akakemie der Wissenschaften zu Berlin (1946–1949), 199–201; W. Heisenberg et al., “Friedrich Pashen,” in Annalen der Physik, 6th ser., 1 (1947), 137–138. A notice by Carl Runge in honor of Paschen’s sixtieth birthday, “Friedrich Paschen,” in Naturwissenschaften, 13 (1925), 133–134, gives reminiscences of the origin of their collaboration in 1895; Niels Bohr, “Friedrich Paschen zum siebzigsten Geburtstag,” ibid.,23 (1935), 73, testifies to Paschen’s “happy intuition, by which he always has pursued experimentally those problems the investigation of which proved to be of decisive significance for the extension of general theoretical conceptions.”