This is a monumental production, in many respects. The copious book of 562 pages is organized in only ten chapters. Accordingly, these are hefty. This, together with the style, whose characteristic is not levity, loaded as it is with both long words and many quotations, makes for a reading a bit difficult at times. However, this is more than compensated by the author’s admirable mastery of his material.The author, who has already several science historical books under his belt, teaches at Göttingen.

After an introductory chapter, Klaus Hentschel starts by putting the sunlight spectrum in historical context. Chapter 3 is devoted to the interplay of representational form and purpose, i.e., selecting a means to an end; such as zooming-in to enlarge a spectral segment of particular interest. The next three chapters focus on techniques of representation, which were all applied to communication of spectral information, engraving and lithography, printing technology, predominantly during the second half of the nineteenth century; and the rise of photographic methods for recording and displaying spectra.  Chapter 7, a mixed bag, deals with photographic emulsions improved by various sensitizer dyes to boost sensitivity in given parts of the spectral range; it covers also intensity measurements (photometry). Chapter 8, probably to the relief of many a utilitarian reader, finally comes to grip  with the use of spectroscopic signatures to identify elements. Hentschel carries out this part of his narrative, in the maybe old-fashioned but effective portrayal of individual  scientists (Hinrichs, Balmer, Rydberg, Lecoq, Ciamician, Harley, Piazzi Smyth, Alexander Herschel, George Higgs, …). Chapter 9 is a very welcome presentation of the first attempts at teaching spectroscopy. The epilogue brings together the various strands, under the overarching metaphor embodied in the title, that of ‘mapping the spectrum,’ which Hentschel urges ought to be taken seriously.

Mapping the spectrum is replete with fascinating material. It will be mined by many historians who, I hope, will be paying proper credit to Dr. Hentschel’s research and erudition. As an example, anyone interested in spectral resolution, will be duly impressed with the ingenuity of Ludwig Becker (1860-1947), who in the latter part of the 1880s estimated line intensities on a continuous strip of paper 314-foot-long, covering the full range from 6024 Å to 4861 Å.

It is only because Dr. Hentschel displays such an impressive and such a minute at times command of his subject matter that some of his misses are all the more irritating. I will mention only two. Whereas he refers to William Playfair (1759-1823) as the inventor of various means of visual representation for scientific data, he fails to mention Johann Heinrich Lambert (1728-1777) who, also in the eighteenth century, arguably had comparable importance. And a truly major lapse in the bibliography is the passing over of the highly relevant books by Edward R. Tufte, The Visual Display of Quantitative Information (1983), Envisioning Information (1990), and Visual Explanations (1997), authentic gems in all three modes of the textual, the iconic and the illustrative, or iconographic.

Which brings me to the quality of the iconography in the book under review. By and large, it is rather outstanding. We have to be extremely grateful, first and foremost to Dr. Hentschel, but also to the publishers, Oxford University Press, and to the Georg-Agricola-Gesellschaft who granted support for inclusion of four color plates. There are no fewer than a total of 144 illustrations—if you are on the lookout for displays of the visible spectrum or of various absorption or emission spectra, take your pick! Rather outstanding, as I wrote: the scale of the reproduction is often way too small for the naked eye to be able to see, let alone read some of the key details. A yet more serious production flaw is the absence of a subject index.

This review will now take up in turn a baker’s dozen of questions one may legitimately ask, with respect to the material covered in this book. The first such question, admittedly philosophical, is whether a spectrum amounts to a set of objective facts. This obviously relates to the present controversies, between scientists as historians (Joseph S. Fruton, for instance) and sociologists of science (such as Steve Shapin or Bruno Latour), regarding the social construction of scientific facts and concepts.

It is most revealing to discover our contemporary debates anticipated in the nineteenth century already. Observers were challenged to record line intensities at least semi-quantitatively from visual impressions, frought with all kinds of subjective bias. Hence, from early on, there was a push for automating such determinations, in order to null the personal equation of the observer.

This trend may have culminated in 1940 with publication of the Utrecht Photometric Atlas of the Solar Spectrum. It made use of a continuous recording made with a microphotometer scanning the original photograph at regular fixed intervals. Yet, even such an automated procedure attracts the objection of bias, from the parsing of the breadth of the scan, in itself only apparently an innocent decision.

A more serious, at least to me, objection questions the purpose of such data collection. If done for its own sake, what merit does it have? I know that I open here a whole can of worms and that I am inviting vehement protest, pointing for instance to the fruitfulness of Johannes Kepler’s theorizing based upon the observations by Tycho Brahe. 

Can mapping the spectrum be construed as one of the earliest and most fruitful applications of photography to science? This reviewer answers with an unqualified "yes."  Records on photographic plates allowed for careful study, complete with re-examination when, at long last, photographers gave themselves the capability to fix images in semi-permanent fashion. Photographic data was congenial to any search for their implicit meaning. What is the point of gathering data, anyhow, if not to hunt for its significance?  Research started with systematizing the gathered data. It then could move on to discovery, to refer only to one of the most productive applications, under the scrutiny of the likes of Johann Jakob Balmer (1825-1898), Theodore Lyman (1874-1954), Friedrich Paschen (1865-1947), of the underlying numerological rules. Which paved the way, in turn, for the triumphant edification of the theory of atomic structure, at the hands of Thomson, Rutherford, and ultimately Niels Bohr.

Credit for this felicitous part of the tale has to go to William Henry Fox Talbot (1800-1877). He has to be singled out: so much was achieved by the English amateur scientist. Already in 1826, he made pioneering observations of absorption lines in the spectra of sodium, potassium and strontium salts held in a flame. His devising the calotypes, i.e., photographs on a paper support which could be duplicated and existed as either positives or negatives (1840), was a major step forward for artistic or scientific photography.

It is a shame that Fox Talbot’s attempt to patent photography—and his going to court in his ill-thought and a priori doomed effort to retain intellectual property—has put his name in a shadow. The best way to look at the mutual misunderstanding between Fox Talbot and his critics is, I submit, in terms of the accelerated professionalization of science around him.

One of his legacies, though, was the "pencil of nature"  metaphor. He thus entitled his first collection of photographs, published in book form in 1844. We may be struck, nowadays, in the naive faith it expresses in the power of photography to register natural phenomena, in their essence and being, with the photographer taking no part in the process. Nevertheless, the formula encapsulates the belief of a whole age; and we remain indebted to it.

One has to note, at this point, that of course the "mapping the spectrum" metaphor runs totally counter to "the pencil of nature." The former trope is anthropocentric, whereas Fox Talbot’s partakes conversely of some vague pantheism.

Edmond Becquerel (1820-1891) was among the first to use photographs to record the solar spectrum. From hindsight, photographic recording techniques would become precious in a variety of scientific fields, especially after the First World War, when the "pencil of nature" would inscribe signatures from evanescent elementary particles, X-ray reflections from crystalline lattices,  or yet very distant stars and galaxies. And, until the advent of photomultipliers, the photographic plate was unrivalled for such tasks. Other recording supports, such as paper, or a drum coated with carbon black, offered little competition.

Which is not to extoll the perfection of photography for recording scientific data. For quite a few applications, such as mapping the spectrum, it presents too narrow a window, in terms of the accessible range of wavelengths; it suffers, moreover, from variable sensitivity throughout the spectral range; likewise, its "dynamic range," to use a current idiom, is also rather severely limited ; the graininess of the emulsion puts a limit on resolution. Not to mention the artefacts, such as solarization, halation, etc.

Nor the ease with which a photograph can be doctored. Fraud, deliberate or inadvertent, is all too easy. Just think of all the portraits of spirits which psychics of the Victorian Age were able to shoot! Arthur Conan-Doyle (1859-1930), a smart person if there ever was one, let himself be sucked in, to mention only one example.

Which foucaldian episteme was "mapping the spectrum" part of? A factor definitely was Comtian positivism. Gathering the facts was felt to be more than a duty, it was deemed the responsibility of scientists to do so. As Hentschel writes (p. 86), "why was it repeatedly felt necessary—on the average, every ten years, between 1814 and 1890—to invest one or more observer years in the meticulous registration of tens of thousands of micrometer readings (with many an observer being forced to stop because of the severe strain on the eyes)? "
Another factor was encyclopedism. As one will recall, the nineteenth century was prolific in the publishing of encyclopedias, for reasons which included the rise of literacy, new printing technologies such as lithography or steelplate engraving, education of the masses and reverence for science. Yet another factor in the collective effort at charting spectra, those of visible light and of the elements, in both emission and absorption, was the urge to understand the wonders of nature: natural history was switching from the descriptive to the explanatory mode. An example, related to the topic at hand, was Lord Rayleigh (1842-1919) providing an explanation for the blue of the sky.
The nineteenth century was an age of exploration. Exploration, as a notion, combines the search for facts, the encyclopedic urge for completeness, and a will to discover all the wonders of the natural world. The nineteenth century, by and large, saw completion of geographic exploration of the planet. Charting the spectrum is coterminous with the charting of the globe, as it had started in the eighteenth century with the French academicians (such as Maupertuis and La Condamine) surveying the shape of the Earth, and as it was brought to a measure of completion during the course of the nineteenth century.

The spectrum, in like manner to the American West, was virgin territory.The Western mind sought to survey such empty spaces, and to record in texts their features. Examples that leap to mind are those of Thomas Jefferson’s conceiving of the Lewis and Clark Expedition, or, about a century later, the literary output of Jules Verne, bearing testimony to this urge for mapping previously unrecorded territories.
Mapping out a virgin space, however, is not intention- or value-free. Neither was "mapping the spectrum" neutral and ideology-free. It could not be. Even in its most modest productions, cartography carries a cosmology with it—for instance, when Leonardo da Vinci mapped the city of Imola, and his rendition incorporated implicitly the Copernican revolution.
Hentschel is rather discreet on such questions, unfortunately.

And what does periodization of the story reveal? Dr. Hentschel starts it in the year 1800, shortly after the invention of lithography, when thermometric and chemical recordings of invisible light were devised. As already stated, photographic techniques became available after 1840. Around 1855, symbolic spectrum representations—as opposed to those termed "iconic" by Hentschel—gained the upper hand. After Gustav Robert Kirchhoff (1824-1887) introduced in 1861 the first numerical scale, Bunsen’s diagrammatic picture, in 1863, became a benchmark. During the 1870s and 1880s, means such as heliography made it possible to convert a photograph into a printable plate. Accordingly, the last decade in the nineteenth century saw a relatively stable type of iconic spectral map. This dominance was once again upturned, when from 1913 until 1925, symbolic term diagrams came to the fore and paved the way for quantum theory and, from 1925 on, for quantum mechanics. However, from 1925 on, one witnesses a reversion to an iconic style of spectrum representation.

Unfortunately, readers are presented with such a chronology at the end of the book only, in the Epilogue. Moreover, this timeline remains descriptive, the author makes no attempt at finding the reasons for such switches between the realistic and the schematic imaging of the spectrum.

One such image became extremely influential. Robert Bunsen (1811-1899) published in 1863 a symbolic plot of emission spectra and continuous spectra. Was it indeed "the most frequently reprinted scientific illustration in the second half of the nineteenth century? (p. 48)" It may have been and, in any case, this picture became one of the most influential representations in scientific iconography. Its role during the nineteenth century may be compared to that of the double helix in the aftermath to Watson and Crick’s discovery. Most spectra were drawn after the general style of Bunsen’s chart. For instance, I have on my shelves a little yearly handbook entitled L’Agenda du chimiste (Paris : Hachette). It is one of the forerunners to the Handbook of Chemistry and Physics. The copy I own is that for the year 1887. Pages 420-424 are given to the display of absorption of spectra for a variety of chemicals, natural dyes predominantly; no fewer than 138 spectra are thus displayed, schematically.

One is thus tempted to leap to the twin conclusions that only a symbolic representation allows scientific accrescence; and that it is indeed a pre-requisite to the advancement of knowledge. Indeed, one might make the claim that Bunsen’s schematic depiction ushered in spectroscopy, because it implied a one-to-one correspondence between an absorption spectrum and the identity of a substance: Bunsen’s representation thus was synonymous with a spectroscopic signature, a notion and a tool both, which chemists in particular were very quick to grab.

A tool? A good example, in a related case, for emission rather than for absorption spectra, is that of the sodium D lines. The two closely spaced sodium Fraunhofer D lines at 589.6357 and 589.0186 nm, natural twins, very intense, turned into a celebrated actor in the history of science and technology. Their use illustrates a key feature of scientific discovery, the near-instant recycling into the toolbox for the scientist.
You throw a little salt into a gas flame, such as that from a Bunsen burner and you see an intense yellow light. A little salt? Only three nanograms of sodium suffice for such visual detection! Furthermore, the two lines are so close in their wavelengths that the radiation is effectively monochromatic: which already enabled scientists, during the second half of the nineteenth century, to use such sodium emission as their light source in order to record, for instance, optical rotations []D with a polarimeter. And, to mention only an altogether different use, the same intense light source allowed engineers to devise sodium vapor lamps, which came into widespread use during the twentieth century, to illuminate highways in particular.

And what goes into the making of a spectroscopist? This is one of the best parts of this book, and I enjoyed it thoroughly: it is most convincing, because of the wealth of biographical documentation with which Hentschel bolsters his case. And it is vividly interesting, since it focuses on graphic art as part of engineering studies, which combines acute visual observation with very careful and minute drawing skills, and which bridges art and science—which in this case provided optimal training to future spectroscopists. These tended to have a family background in the arts and crafts (engravers, printers, lithographers). Their studies took them to a Polytechnic, or to a similar institution putting emphasis on acquisition of such skills as technical drawing, draughtsmanship, perspective, descriptive geometry, and so on. Often, they went on to themselves become instructors at such technical schools. Furthermore, their drawing prowess became as much an avocation as it was a nurtured talent. They would give themselves hobbies to which this particular expertise could be put. Hence, «mapping the spectrum» came naturally to such persons.
Not only is this, arguably, one of the best segments in the book, it also deconstructs two of Hentschel’s key contentions. He argues most forcefully and at some length that spectroscopy does not amount to a scientific discipline. Yet, in offering such a convincing portrayal of the education of a spectroscopist, he implicitly assumes the existence of such a category, of such a social group as a subset within the larger class of scientists.
That the making of spectroscopists was, historically speaking, so much determined by draughtsmanship, also ruins the central «spectrum-as-a-map» metaphor. If indeed we follow this tantalizing hint of Hentschel’s, as to the education of spectroscopists, then we should expect such a training of spectroscopists as draughtsmen to translate into production of plots, graphs, blueprints, working drawings, diagrams, etc., rather than maps. Cartography was the province of artistic rather than technical illustrators.
At this point, I would only interject that a more adequate designation of the spectrum, rather than as a map, would have been as a pattern. A map can be defined as an image, with an edge or contour enclosing a solid two-dimensional space, and bearing names, directly or indirectly. Map contours have inherent self-similarity. Maps, also and as a rule, have a scale. And maps, as quite a few contemporary authors have pointed out, are theory- or ideology-laden. Representations of spectra do not show those features of maps, which are also absent from patterns. The latter are fractality-free, metric-free and ideology-free. A joint characteristic of maps and patterns is that they can be learned and thus become recognizable.
Even though Hentschel’s book is disappointing in that it fails to develop his "spectrum-as-map" metaphor, nevertheless this analogy puts a stranglehold on all other metaphors, which are at least as productive. I shall mention only two, considerably more helpful, in this reviewer’s opinion. The format of this review does not leave room for any fuller treatment.
Hentschel would have been well advised to investigate art history. It is astounding to this reviewer that a book, dealing explicitly with visual representations, would fail to seek analogs, precedents and influences, in the artistic sphere.     
A key artistic genre, from which Bunsen’s 1863 diagrammatic depiction of the spectrum obviously derives, is the silhouette. It was in great vogue at the end of the eighteenth century and during the first half of the nineteenth century, when photography dethroned it from its eminence. Some of the artists who introduced—or rather reintroduced—such profiles in shadow were, during the Georgian Age, the British artists Isabella Beetham (1753-1825), Charles Christian Rosenberg (1745-1844), John Miers (1756-1821), and the Frenchman Augustin Edouart (1789-1861). These were all practitioners of an art (and a pastime) which Robert Bunsen was very familiar with. 

I can do no better, at this point, than to quote Francis Galton (1822-1911) who provided this capsule description of silhouette portraits in a letter to The Photographic News (July 8 1887, pp. 429-430; see also p. 462):

"black silhouettes … were very familiar to those who lived in the pre-photographic period. They were quickly cut out of paper by a deft hand with a small keen pair of scissors, and at least one of the many operators in this way ranked as an artist capable of making excellent likenesses. The paper was black on one side, and the silhouette that had been cut out was pasted then and there, with the black side upwards, upon a white card, and framed. A perfectly durable, and often a good likeness was thereby produced in a very short time. "

The following year, 1888, Galton published an article in Nature describing profile data and its use in the context of face recognition, and of the anthropometric identification of an individual, which he was pioneering ; Robert Bunsen was familiar with silhouette drawings, because they were part of the visual culture of the time.
What is a visual culture? It consists of both a lexicon and a syntax. The lexicon is a repertory of signs and images. For instance, in our time, such lexicon includes logos for cars, clothing manufacturers, other businesses. In other words, a visual culture is built from a symbolic language, made to a significant extant from iconic stereotypes. The means by which these were both communicated and reinforced were, at the time when Bunsen developed his shorthand for spectra—let me only note in passing that the invention of shorthand writing would have been yet another fruitful metaphor, perhaps more so than the map—, newsprint predominantly: newspapers, magazines, and books. As is well known, these were abundant during the Victorian Age. Moreover, printing techniques were used, besides text, for reproductions of images, from steelplate engravings, lithographs, and the first photographs. Furthermore, there was also an abundance of images to be looked at, within the fast growing social class of bourgeois city dwellers, as paintings, artistic prints, slides mounted between two glass plates, stereoscopic or kaleïdoscopic views, and so on and so forth. Silhouettes were an integral part of the visual culture of the age, and it was thus natural for Bunsen to turn to them for his 1863 presentation of the spectrum.
The second metaphor, which I argue is also more useful than that of the map, one which is still very much with us, is that of the fingerprint. We even keep referring to parts of say an infrared spectrum as the "fingerprint region." With this metaphor, avowedly anthropomorphic, we convey the notion of a one-to-one correspondence between a spectrum and the chemical system which it serves to identify.
Sir Francis Galton, whom I have just quoted with respect to silhouette portraits, and with the same motivation of individual identification among people, became strongly interested in fingerprints in the 1880s and published a book entitled Fingerprints in 1892. Spectral representation in print and the imprint of a person thus enjoyed contemporary heydays.

And what was the date of birth for the notion of fingerprinting a chemical with a spectrum? One of the surprises, to this reviewer, was how far back in the nineteenth century one ought to look for the birth of chemical spectroscopy. Once again, Fernand Braudel’s intuition of history making sense primarily in la longue durée is vindicated by the evidence ! Already in the mid-1830s, William Henry Fox Talbot could write percipiently (as quoted by Hentschel, p. 45) :

"the strontia flame exhibits a great number of red rays well separated from each other by dark intervals, not to mention an orange and a very definite bright blue ray. The lithia exhibits one single red ray. Hence I hesitate not to say that optical analysis can distinguish the minutest portions of these two substances from each other with as much certainty, if not more, than any other known method."

Soon thereafter, an outsider (he was an American physician), David Alter (1807-1881) published the emission spectra from various metals in the electrical arc. Later on, after 1860, he would claim the discovery of spectral analysis, with rather good justification. Unfortunately, as Hentschel notes on p. 46, "physicists generally were not yet interested in problems of chemical analysis."

The 1830s were also the time when Auguste Comte (1798-1857) peremptorily asserted, in 1834:

"There are some things of which the human race must forever remain in ignorance, for example the chemical constitution of the heavenly bodies."

Following the dramatic entry of spectral analysis in 1859-60, solar and stellar chemistries came into being, thus showing as ludicrous the dogmatic pronouncement by the French philosopher (many such a statement of impossibility has likewise been turned into a joke by subsequent scientific or technological history).  
    The other major historical contribution of spectroscopy to chemistry was, of course, toward establishment of atomic structure. The initial discovery, in 1883, was Alexander Stewart Herschel (1836-1907), a member of the famous scientific dynasty of astronomers—he was the grandson of William Herschel. He found the recurring pattern among spectral lines. And his enthusiasm is worth quoting:

"And then to TRIUMPH! In searching over the spaces of my ‘readings’ to identify your lines with, [he is writing to Charles Piazzi Smyth] I lighted luckily on the key to the construction, which is simplicity itself, and couldn’t well be exceeded in the exactness with which your new map reveals it. Lux in tenebris, what a happy and glorious release you have disclosed to all our uncertainties!"

This finding was quickly given its analytic expression in equation form, the so-called geometric law (p. 320). The Herschels were musicians and scientists, it should come as no surprise if one of them discovered such an inner harmony in matter.
    The next step belonged to an outsider, Johann Jakob Balmer (1825-1898). Balmer was neither a physicist nor a chemist, he was a Swiss mathematics schoolteacher. He interpreted the lines in the hydrogen spectrum as optical harmonics. As Hentschel writes most perceptively, Balmer’s insight came from his having "the eye of a geometrician used in teaching the rules of perspective. "  (p. 296). Balmer did not content himself with the numerology implied by the line frequencies, in a series. He gave himself a two-dimensional geometric reconceptualization. He saw an analogy between the regularly decreasing spaces separating the spectral lines, on one hand, and the "pattern [pattern, not map!] formed by the railway ties of a train track" (p. 442) receding in the distance, from the eye of an observer. Hentschel is right to emphasize that Balmer’s discovery of the series formula for hydrogen was made in the context of perspectival drawing, a mental leap for which he was uniquely qualified.
    The discoveries by Herschel and Balmer answer the question of interpretation. They translate a spectrum into meaning, i.e., into a set of asserted propositions, which can be subsumed into a mathematical formula of Pythagorician simplicity, and thus of arresting beauty.

Balmer, the schoolteacher, is an endearing figure in this narrative. But he is not alone. Arguably, in a popularity contest within the narrow circle of pioneer spectroscopists, in contention with the likes of Robert Bunsen and Gustav Kirchhoff, who also have most interesting biographies, the palm should nevertheless be awarded to Joseph Fraunhofer (1787-1826). He had very modest origins, he was the son of a glazier. For quite a few years, he was a workman, cutting glass. This self-made man gave himself training in physics and in mathematics. By 1811, Fraunhofer became the director of an Optical Institute in Bavaria, which made large quantities of bubble- and striae-free achromatic glass, for instruments such as theodolites and telescopes. He perfected or invented quite a few optical instruments. His work on the solar spectrum, which he was able to observe combining a prism and a narrow slit, culminated with his publication of a copper engraving he himself made (1814). He had discovered the dark lines in the solar spectrum. Later on (1821), he invented diffraction gratings. Germany’s subsequent supremacy in optics, in the design and manufacture of optical instruments, owes him much.

To sum up: Dr. Hentschel has gathered an outstanding documentation; the central thesis in his book is debatable, and one may regret that, perhaps influenced by some current fads, there is no more body to the narrative presented. Both the topic and the readership would have deserved it.

Klaus Hentschel Mapping the Spectrum. Techniques of visual representation in research and teaching.
Oxford : Oxford University Press, 2002.
ISBN 0-19-850953-7. xii, 562 pp., illus.,, index

Review for Foundations of Chemistry