The Net Advance of Physics RETRO:
Part One

Gymnasium Kreuzgasse, Cologne -- a modern high school descended from the one where Schellen taught. Photo by "7Rabru7".

Popularisation of science is a thankless task. Popularisers and educators are the only people standing between scientific civilisation and the proverbial ever-returning Dark Ages; it is they alone who steer the commonwealth away from both the Scylla of rejecting science as incomprehensible and the Charybdis of accepting it on blind faith. Nevertheless, their supposed lack of "originality" means that they have less prestige in the scientific community than mediocre or even incompetent "researchers". Boston University once famously deemed Isaac Asimov an underperforming biochemist ; in these supposedly more enlightened times, many reviewers of Martin Gardner's recently-published posthumous memoirs seem compelled to assert that the man Donald Knuth has called the most influential mathematics teacher "in the history of the world" was not actually a "real" mathematician.

Of course, some popularisers are at least household names in their own lifetime. Their memory, however, fades quickly, and all the more so if they wrote in a language the importance of which has diminished with time. Such was the fate of the subject of this week's blog: the once-famous "Dr. H. Schellen", whose account of the nebulæ we will be reading in a moment. But who was the man himself? Even in German, there is not much about him online, and in English there is almost nothing ; to fill this gap, I have translated the following entry (by Karl Ernst Hermann Krause) from the standard reference-work Allgemeine Deutsche Biographie [Munich: Duncker & Humblot, 1890], volume 30, page 761.

SCHELLEN, Thomas Joseph Heinrich, born 30 March 1818 at Kevelaer, Düsseldorf administrative district; † 3 September 1884 at Cologne.

After completing his studies, he spent his probationary initial year as a teacher at the Friedrich-Wilhelmsgymnasium in Cologne in 1841; at the same time, he was active at the Municipal Trade-School there. As early as 1842 he was hired as a faculty member of the Düsseldorf Technical School [Realschule -- a kind of high school, but there is no exact Anglo-American equivalent], in which position he remained until 1851. In that year, a technical school and associated municipal trade-school were founded in Münster, and Schellen, whose unusual capacity in technical-school matters was known, was appointed director of the new establishment, which would flourish under his energetic leadership. Here he worked for seven years, after which in 1858 he was called back to Cologne to fill the position of director for the Höheren Bürgerschule (the present [i.e. 1890] Technical High School). Schellen administered this institution in an exemplary way until his retirement in 1881.

Besides his arduous profession as a teacher and administrator, Schellen was extraordinarily active as a writer, and through this his robust health was broken. Repeated strokes urgently reminded him first to reduce work in his beloved occupation, and soon to abandon it entirely -- an exceedingly difficult resolution for one so mentally active.

How hard Schellen worked all his life, and what he thereby contributed to education and the advancement of science, are shown by the great number of his scientific writings, many (indeed, almost all) of which have run through numerous editions. Besides his publications with mathematical and physical content for educational and scientific journals are some works intended chiefly for practical use at the technical school, involving only arithmetic, which let us perceive Schellen's pædagogic ability : (Methodisch geordnete Materialien für den Rechenunterricht, ein Handbuch für Lehrer, nine editions; Elementare Mechanik, in two volumes, four editions). Moreover, through his writings, Schellen became well-known in a wide circle for his outstanding ability to explain the major advances of science and technology in a popular, yet still scientific, way. Notable among such works are Der elektromagnetische Telegraph, (six editions); Das transatlantische Kabel, seine Fabrikation, Legung und Sprechweise; Die Spektralanalyse in ihrer Anwendung auf die Stoffe der Erde und die Natur der Himmelskörper (three editions); Die Sonne, (a stand-alone revision of the celebrated work by Secchi); Das Spektroskop, (adapted from the writings of Lockyer); and Die magnetischen und dynamoelektrischen Maschinen, ihre Entwicklung, Construktion und praktische Anwendung, (three editions).

These books contained no original investigations of Schellen's, but their significance for the advancement of science should not be underestimated. Most of these works dealt with areas just developed, where advances and new discoveries had to be recorded daily. In every succeeding revised edition the newest positions attained by research, science, and practice were correspondingly set forth, so that each edition stood on the cutting edge of its time and was not so much a reworking as almost a new book in its own right.

By his clear, easy language, Schellen made a most uncommon contribution, making these remarkable discoveries and inventions understandable to the broadest public.

The following article on Nebulæ gives a good idea of Schellen's style, not to mention the wild worlds of Nineteenth Century international publishing and Twenty-first Century Internet archiving. The German base-text is Spectra der Nebelflecke und Sternhaufen, section 59 of Die Spectranalyse [Brunswick: Westermann, 1870]. (The scan is of the first edition.) The complete second edition of Spectranalyse was translated into English by Jane and Caroline Lassell under the title Spectrum Analysis [New York: Appleton, 1872]. (WARNING: the link is to a typically unreadable Google scan, with inadvertently missing pages etc. as usual. Unfortunately no non-Google scan presently exists at the Archive. Another Google scan of the same text has all the pages, but seems to have been photographed through a distorting lens!) The text below, which appeared without a translator's name in Popular Science 3, 129 (1873 June), is an abridgement of the nebula section of the Lassells' translation, omitting mostly tables of data. The typesetter seems to have misunderstood which captions go to which figures (I have corrected this), and the text breaks off abruptly, as though the Popular Science editors had run out of space. I have therefore supplied a longer ending from a different uncredited snippet of the Lassells' version: Spectrum Analysis Explained, compiled by the Editor of "Half-Hour Recreations" from the Works and Observations of Prof. Schellen and Others, [Boston: Estes and Lauriat, 1872].

(The "Editor of Half-Hour Recreations" was Dana Estes, one of the first American publishers to issue mass-market editions of serious literature. As we read in the 1874 December 4 Harvard Crimson: "Few college men there are but would like to read and own many capital books, but are deterred from buying by the $2.50 regular price, even with a mysterious 'trade,' 'cash,' and 'personal favor' deduction reducing it to $1.13 ... Could the piracy so indiscriminately employed with the books of English authors be turned to some public good, the school-boy of the future might buy Tom Brown for a dime, and the poorest family might have its Bible, Shakspere, and Principles of Political Economy ... Something of the kind has been done by that enterprising house Messrs. Estes and Lauriat ... Americans want something more than ephemeral newspaper literature, and the time has come for cheap books. When the best works of the standard authors can be bought at moderate prices, men of moderate means will not be slow in buying freely.")

Figure 1: The Great Nebula in Orion.


by Dr. H. Schellen

When the starry heavens are viewed through a telescope of moderate power, a great number of stellar clusters and faint nebulous forms are revealed against the dark background of the sky which might be taken at first sight for passing clouds, but which, by their unchanging forms and persistent appearance, are proved to belong to the heavenly bodies, though possessing a character widely differing from the point-like images of ordinary stars.

Sir William Herschel was able, with his gigantic forty-foot telescope, to resolve many of these nebulæ into clusters of stars, and found them to consist of vast groups of individual suns, in which thousands of fixed stars may be clearly separated and counted, but which are so far removed from us that we are unable to perceive their distance one from the other, though that may really amount to many millions of miles, and their light, with a low magnifying power, seems to come from a large, faintly-luminous mass. But all nebulæ were not resolvable with this telescope, and, in proportion as such nebulæ were resolved into clusters of stars, new nebulæ appeared which resisted a power of 6,000, and suggested to this astute investigator the theory that, besides the many thousand apparent nebulæ which reveal themselves to us as a complete and separate system of worlds, there are also thousands of real nebulæ in the universe composed of primeval cosmical matter out of which future worlds were to be fashioned.

Lord Rosse, by means of a telescope of fifty-two feet focus, of his own construction, was able to resolve into clusters of stars many of the nebulæ not resolved by Herschel; but there were still revealed to the eye, thus carried farther into space, new nebulæ beyond the power even of this gigantic telescope to resolve. Telescopes failed, therefore, to settle the question whether the unresolved nebulæ are portions of the primeval matter out of which the existing stars have been formed; they leave us in uncertainty as to whether these nebulæ were masses of luminous gas, which in the lapse of ages would pass through the various stages of incandescent liquid (the sun and fixed stars), of scoriæ or gradual formation of a cold and non-luminous surface (the earth and planets), and finally of complete gelation and torpidity (the moon), or whether they exist as a complete and separate system of worlds; telescopes have only widened the problem, and have neither simplified nor solved its difficulties.

That which was beyond the power of the most gigantic telescopes has been accomplished by that apparently insignificant, but really delicate, and almost infinitely sensitive instrument -- the spectroscope. We are indebted to it for being able to say with certainty that luminous nebulæ actually exist as isolated bodies in space, and that these bodies are luminous masses of gas.

The splendid edifice already planned by Kant in his Theory of the Heavens (1755), and erected by Laplace forty-one years later, in his System of the Universe, has received its topmost stone through the discoveries of the spectroscope. The spectroscope, in combination with the telescope, affords means for ascertaining even now some of the phases through which the sun and planets have passed in their process of development or transition from masses of luminous nebulæ to their present condition.

Great variety is observed in the forms of the nebulæ: while some are chaotic and irregular, and sometimes highly fantastic, others exhibit the pure and beautiful forms of a curve, a crescent, a globe, or a circle. A number of the most characteristic of these forms have been photographed on glass at the suggestion of Mr. Huggins; to these have been added a few others, taken from accurate drawings by Lord Rosse; and they may all be projected on to a screen by means of the electric or lime-light lantern, and made visible to a large audience.

Figure 2: Central and Most Brilliant Portion of the Great Nebula in the Sword-Handle of Orion, as Observed by Sir John Herschel in his Twenty-Foot Reflector at Feldhausen, Cape of Good Hope, (1834 to1837). South is at Top.

The largest and most irregular of all the nebulæ is that in the constellation of Orion (Figs. 1, 2). It is situated rather below the three stars of second magnitude composing the central part of that magnificent constellation, and is visible to the naked eye. It is extremely difficult to execute even a tolerably correct drawing of this nebula; but it appears, from the various drawings made at different times, that a change is taking place in the form and position of the brightest portions. Fig. 2 represents the central and brightest part of the nebula. Four bright stars, forming a trapezium, are situated in it, one of which only is visible to the naked eye. The nebula surrounding these stars has a flaky appearance, and is of a greenish-white color; single portions form long curved streaks stretching out in a radiating manner from the middle and bright parts.

Figure 3: The Large Magellanic Cloud.

Much less irregularity is apparent in the great Magellanic or Cape clouds (Fig. 3), which are two nebulæ in the Southern Hemisphere, one of them exceeding by five times the apparent size of the moon. They are distinctly visible to the naked eye, and are so bright that they serve as marks for reconnoitring the heavens, and for reckoning the hour of the night.

Figure 4: Nebula of the Form of a Sickle.

The interest aroused by these irregular and chaotic nebulous forms is still further increased by the phenomena of the spiral or convoluted nebulæ with which the giant telescopes of Lord Rosse and Mr. Bond have made us further acquainted. As a rule, there stream out from one or more centres of luminous matter innumerable curved nebulous streaks, which recede from the centre in a spiral form, and finally lose themselves in space. Fig. 4 represents a nebula in the form of a sickle or comet-tail, and Fig. 5 (below) shows the most remarkable of all the spiral nebulæ, situated in the constellation Canes Venatici.

Figure 5: Spiral Nebula in Canes Venatici.

It is hardly conceivable that a system of such a nebulous form could exist without internal motion. The bright nucleus, as well as the streaks curving round it in the same direction, seems to indicate an accumulation of matter toward the centre, with a gradual increase of density, and a rotatory movement. But, if we combine with this motion the supposition of an opposing medium, it is difficult to harmonize such a system with the known laws of statics. Accurate measures are, therefore, of the highest interest for the purpose of showing whether actual rotation or other changes are taking place in these nebulæ; but, unfortunately, they are rendered extremely difficult and uncertain by the want of outline, and by the remarkable faintness of these nebulous objects.

Figure 6: Transition from the Spiral to the Annular Form.

The transition state from the spiral to the annular form is shown in such nebulæ as the one represented in Fig. 6; and they then pass into the simple or compound annular nebula of a type which is given in Fig. 7 (below).

Figure 7: Annular Nebula in Lyra.

The space within most of these elliptic rings is not perfectly dark, but is occupied either by a diffused nebulous light, as in Fig. 7, or, as in most cases, by a bright nucleus, round which sometimes one ring, sometimes several, are disposed in various forms.

Figure 8: Nebula with Several Rings

In Fig. 8 a representation is given of a compound annular nebula, with very elliptic rings and bright nucleus.

Figure 9: Elliptical Annular Nebula

According as the ring has its surface or its edge turned toward us, or according as our line of sight is perpendicular or more or less obliquely inclined to the surface of the ring, its form approaches that of a circle, a ring, an ellipse, or even a straight line. Nebulæ of this latter kind are represented in Fig. 9 and in Fig. 10 (below).

Figure 10: Elongated Nebula

When an elliptical ring is extremely elongated, and the minor axis is much smaller than the major one, the density and brightness of the ring diminish as its distance from the central nucleus increases; and this takes place to such a degree sometimes, that at the farthest points of the ring, the ends of the major axis, it ceases to be visible, and the continuity seems to be broken. The nebula has then the appearance of a double nebula, with a central spot as represented in Figs. 11, 12, (below).

Figure 11 (Left): Double Nebula. Figure 12 (Right): Annular Nebula with Centre.

Those nebulæ, which appear with tolerably sharply-defined edges in the form of a circle or slight ellipse, seem to belong to a much higher stage of development. From their resemblance to those planets which shine with a pale or bluish light, they have been called planetary nebulæ; in form, however, they vary considerably, some of them being spiral and some annular.

Figure 13: Planetary Nebula with Two Stars.

Figure 14 (Left): Planetary Annular Nebula with Two Stars. Figure 15 (Right): Planetary Nebula.

Some of these planetary nebulæ are represented in Figs. 13, 14, 15. The first has two central stars or nuclei, each surrounded by a dark space, beyond which the spiral streaks are disposed; the second has also two nuclei, but without clearly separable dark spaces; the third is without any nucleus, but shows a well-defined ring of light.

Figure 16 (Left): Planetary Nebula. Figure 17 (Right): Stellar Nebula.

The highest type of nebulæ are certainly the stellar nebulæ, in which a tolerably well-defined bright star is surrounded by a completely rounded disk or faint atmosphere of light, which sometimes fades away gradually into space, at other times terminates abruptly with a sharp edge. Figs. 16 and 17 exhibit the most striking of these very remarkable stellar nebulæ: the first is surrounded by a system of rings like Saturn, with the thin edge turned toward us; the second is a veritable star of the eighth magnitude, and is not nebulous, but is surrounded by a bright luminous atmosphere perfectly concentric. To the right of the star is a small dark space, such as often occurs in these nebulæ, indicating, perhaps, an opening in the surrounding atmosphere.

Figure 18: Spectrum of Nebula (from Herschel's Catalogue, No. 4,374).

We have now passed in review all that is at present known of the nebulæ, so far as their appearance and form have been revealed by the largest telescopes. The information as yet furnished by the spectroscope on this subject is certainly much less extensive, but is nevertheless of the greatest importance, since the spectroscope has power to reveal the nature and constitution of these remote heavenly bodies.

It must here again be remembered that the character of the spectrum not only indicates what the substance is that emits the light, but also its physical condition. If the spectrum be a continuous one, consisting of rays of every color or degree of refrangibility, then the source of light is either a solid or liquid incandescent body; if, on the contrary, the spectrum be composed of bright lines only, then it is certain that the light comes from luminous gas; finally, if the spectrum be continuous, but crossed by dark lines interrupting the colors, it is an indication that the source of light is a solid or liquid incandescent body, but that the light has passed through an atmosphere of vapors at a lower temperature, which by their selective absorptive power have abstracted those colored rays which they would have emitted had they been self-luminous.

When Huggins first directed his telescope in August, 1864, to one of these objects, a small but very bright nebula, he found, to his great surprise, that the spectrum, instead of being a continuous colored band, such as that given by a star, consisted only of three bright lines.

This one observation was sufficient to solve the long-vexed question, at least for this particular nebula, and to prove that it is not a cluster of individual, separable stars, but is actually a gaseous nebula, a body of luminous gas. In fact, such a spectrum could only be produced by a substance in a state of gas; the light of this nebula, therefore, was emitted neither by solid nor liquid incandescent matter, nor by gases in a state of extreme density, as may be the case in the sun and stars, but by luminous gas in a highly-rarefied condition.

Figure 19: Spectrum of Nebula Compared with the Sun and Some Terrestrial Elements.

In order to discover the chemical nature of this gas, Huggins followed the usual methods of comparison, and tested the spectrum with the Fraunhofer lines of the solar spectrum, and the bright lines of terrestrial elements. A glance at Fig. 19 will show at once the result of this investigation. The brightest line (1) of the nebula coincides exactly with the brightest line (N) of the spectrum of nitrogen, which is a double line. The faintest of the nebular lines (3) also coincides with the bluish-green hydrogen hue Hβ, or, which is the same thing, with the Fraunhofer line F in the solar spectrum. The middle line (2) of the nebula was not found to coincide with any of the bright lines of the thirty terrestrial elements with which it has been compared; it lies not far from the barium line Ba, but is not coincident with it.

[The Popular Science article cuts off here ; the version edited by Dana Estes omits parts of the paragraph above and continues as follows:]

During the years 1865 and 1866, more than sixty nebulæ were examined by Huggins with the spectroscope, mainly with the intention of ascertaining whether those which were clearly resolvable by the telescope into a cluster of bright points, gave a continuous spectrum, or one composed of bright lines.

As a result of his observations, Huggins divides the nebulæ into two groups :

  1. The nebulæ giving a spectrum of one or more bright lines.
  2. The nebulæ giving a spectrum apparently continuous.
About a third of the sixty nebulæ observed belong to the first group ; their spectrum consists of one, two, or three bright lines ; a few showing at the same time a very narrow, faint, continuous spectrum.

The great nebula of Orion has been the subject of spectroscopic investigations. Its spectrum consists of three very conspicuous bright lines, one of which again indicates nitrogen and another hydrogen.

Huggins has lately repeated his former observations with instruments of much greater power, and compared especially these two lines with those of the terrestrial gases, under circumstances which gave him a spectrum four times the length of the one he obtained in his earlier investigations. The result of these observations, continued for several nights, was to show the complete coincidence, even in this greatly extended spectrum of the nebular lines, with those of both gases, so that there can be no remaining doubt as to the identity of the lines.

Half of the nebulæ giving a continuous spectrum have been resolved into stars, and about a third more are probably resolvable ; while of those yielding a spectrum of lines, not one has been certainly resolved by Lord Rosse. Considering the extreme difficulty attending investigations of this kind, there is scarcely any doubt that there is a complete accordance between the results of the telescope and spectroscope ; and therefore those nebulæ giving a continuous spectrum are clusters of actual stars, while those giving a spectrum of bright lines must be regarded as masses of luminous gas, of which nitrogen and hydrogen form the chief constituents.

[If Schellen the great reviser had survived to the present day, how much of the above text would he include in the 2014 edition? I will present a modernised version in Part II, to be posted later in the year.]