CHAPTER 6—PART 1

SOLAR OBSERVATIONS AT TULSE HILL

From time to time throughout his career, William Huggins observed the sun. He had even played a significant role, as we saw in chapter 2, in resolving what came to be known as the "willow-leaves" controversy in the early 1860s, a heated dispute over the appearance and nature of the solar surface.¹  In general, however, solar observation had not netted Huggins any prize discoveries or their attending prestige.  If anything, it had proven frustrating for him.

Recall from chapter 5 that some amateur astronomers, notably Alexander Strange, had publicly criticized Huggins for his failure to incorporate regular and systematic solar observations in his research agenda.  In Strange's view this failure meant Huggins was neglecting his obligation to use the equipment on loan from the Royal Society for the greatest good of the nation.  But, as we saw in chapter 3, routine solar observation had not been the charge given Huggins as part of the terms of the telescope loan, a loan which Strange himself had sanctioned as a member of the Royal Society's Council.  The Royal Society had presented the Grubb telescope to Huggins to pursue his agenda of stellar and nebular spectroscopy, an agenda to which Thomas Romney Robinson had referred at the time with great patriotic zeal.

There were, nevertheless, significant episodes in Huggins career, other than the willow-leaves controversy, during which solar observation became a principal object of his personal agenda.  The outcomes of Huggins' solar research efforts did not always match his expectations, however, and they are conspicuously missing from his retrospective account, "The New Astronomy."²  Huggins' biographers and historians of science have relied heavily on this essay for details of his life and career with the important consequence that Huggins has been remembered chiefly for his stellar and nebular spectroscopic research, while his solar research efforts have been all but forgotten.³

In this chapter, I shall bring to light three of William Huggins' unsuccessful forays into solar observation in order to give a more rounded picture of his endeavors.  I begin with his vain efforts in the late 1860s to observe solar prominences without an eclipse using both filtering techniques and innovative spectroscopic means.  Next, I examine in some detail the solar eclipse expedition to Algeria in December 1870, which Huggins led, and which was unfortunately clouded out.  Finally, I describe Huggins' lengthy efforts to photograph the solar corona without an eclipse, a challenging project he was motivated to pursue largely in consequence of the disappointment he experienced in Algeria.⁴

I shall argue that while solar observation was neither Huggins' forte nor his primary research interest, he was occasionally provoked by priority concerns to contribute to contemporaneous efforts to observe the sun spectroscopically just as solar observers like Lockyer were also drawn to stellar and nebular investigation. In the 1870s and 1880s, the discipline of astronomical physics was developing on many fronts and no one knew which would prove the most fruitful.  Ambitious and competitive pioneers like Huggins and Lockyer pressed the spectroscope into service in a variety of ways searching for discoveries and the recognition they promised.

Huggins' diverse investigations are documented in the pages of contemporary scientific journals.  But in his retrospective essay, Huggins eliminated those projects from his account which had either failed or did not fit directly into the story of the development of stellar spectroscopy.  The unpublished record brings these forgotten episodes to light once again, revealing how Huggins was motivated to modify his research methods and instrumentation in response to failure, as well as how he attempted to convince his colleagues of the validity of his observations in the face of doubt.  As we shall see in the pages that follow, Huggins' solar research efforts, despite their lack of success, played an important role in the development of method and theory in early solar physics.

The Red Flames

I briefly introduced, in chapter 3, Huggins' efforts to view the solar prominences without an eclipse as an example of his eclectic research program in the late 1860s.  In this section, I shall describe his work on the prominences in more detail, this time in the context of Huggins' growing awareness of the need to establish and carefully maintain his priority.

Solar prominences, or "red flames" as they were often called, had attracted the attention of solar eclipse observers since they were first described by Francis Baily after the total solar eclipse in 1842.⁵  Being limited to momentary glimpses of the red flames by the brevity of totality and the capriciousness of the attending weather made it difficult to obtain confirmatory observations.⁶  In 1860, Warren De La Rue had claimed success in photographing the near-solar atmosphere during totality.  He interpreted the images he had obtained as showing the limb of the moon sequentially occulting the flame-like protrusions, and thus convinced his fellow astronomers that the prominences were solar in origin rather than transient features in the terrestrial atmosphere or simply illusions brought on by the sharp contrast of dark and light.⁷  By the latter part of that decade, interest in the nature of solar prominences was again on the rise.

J. Norman Lockyer, for example, whose earlier astronomical interests had centered on the moon and the planet Mars, had turned his attention to the sun in the mid-1860s in part because of the excitement generated by the willow-leaves controversy.⁸  In March 1866, Lockyer began a spectroscopic study of sunspots using a clever method of his own design.  He projected the sun's image onto a screen with a slit.  The screen could be moved to position the slit across a sunspot. In this way a linear segment of the sunspot as well as a portion of the adjoining photosphere was thrown into the attached spectroscope allowing a comparison to be made of the two contiguous spectra.  An analysis of these observations formed the basis of his first paper submitted to the Proceedings of the Royal Society.⁹

Lockyer's paper entitled, "Spectroscopic Observations of the Sun," was communicated on his behalf by the Society's Secretary, William Sharpey on 10 October 1866, and read before the Royal Society at the 15 November meeting.  As we shall see, the dates are significant in terms of the dynamics of Huggins' and Lockyer's competitive relationship and the matter of priority in conceiving a plan to observe solar prominences without an eclipse.  Because of the priority dispute which later erupted over spectroscopic observation of prominences, the findings on sunspots Lockyer reported in his paper did not have the same enduring impact as his final suggestions for possible future applications of the spectroscope to solar research.  In particular, Lockyer's query, "and may not the spectroscope afford us evidence of the existence of the 'red flames' which total eclipses have revealed to us in the sun's atmosphere; although they escape all other methods of observation at other times?" later became a central point of contention.¹⁰  At the time he submitted the paper, Lockyer explained that his spectroscope did not provide him with sufficient dispersing power to render the prominence spectral lines visible without an eclipse.  He successfully applied to the Government Grant Committee of the Royal Society for funds to purchase a more suitable instrument.¹¹

Nine of Huggins' 51 observations recorded in his notebook during 1866 were of the sun.  Recall from chapter 3 that 1866 was a year in which Huggins devoted time to a wide range of projects:  the nova in Corona Borealis, the great meteor shower, nebulae, and variable stars.  On the occasions that Huggins recorded observations of the sun, he reported examining surface features such as sunspots and granulation.  On 26 April, for example, he organized his notebook record as if he were starting a regimen of sunspot observations.  He divided his notes into headed categories like "distribution," "form," "size," and "brightness."  There is no other recording like this in the observatory notebooks in the Wellesley collection.

On 10 November 1866, one day after the regular monthly meeting of the Royal Astronomical Society, and not quite a week before the Royal Society meeting at which Lockyer's paper on solar spectroscopy was to be read, William Huggins wrote in his observatory notebook, "I tried a new method of endeavouring to see the red-flames" by a method that "had appeared to me probable (for some weeks)."¹² The parenthetical remark here is telling.  But to whom was it addressed?  Huggins intended this entry to stand as evidence of priority in the quest for viewing the elusive solar prominences without an eclipse.  Clearly, he anticipated competition.

The method Huggins believed would render the prominences wholly visible to an observer and which he was then claiming to have been piecing together in his mind for some time is notable both for its originality and for the fact that it was not spectroscopically based.  Huggins reasoned that if, as reported, the prominences were red in color, it should be possible to filter out most other regions of the solar spectrum using a stack of differently colored pieces of glass held together with Canada balsam.  When he finally tried this approach in November 1866, however, it proved unsuccessful and Huggins seems to have dropped the project altogether for a time.  If the method for viewing the prominences had been in his mind "for some weeks" already, what motivated him to put it into action at this particular time? Perhaps Lockyer revealed something in informal conversation at the RAS meeting the night before to alert Huggins of his own interest in the matter.¹³

In August 1868, a total solar eclipse of over five minutes duration was predicted to cross a number of favorable sites in India and Malaya.¹⁴  Plans were made in Britain to mount expeditions to observe it.  By early 1867, in anticipation of the upcoming eclipse, George Stokes began gathering suggestions from his astronomer colleagues on ways of effectively analyzing the light from the red flames with a spectroscope.  Sir John Herschel sent some helpful advice.  In fact, he became so enthused with his own thoughts on the matter that he proceeded to suggest how such a method might be extended to permit observations of the solar prominences without an eclipse.  Herschel believed prominence spectra might prove to be so distinctive that it would be possible to detect their presence simply by comparing the spectrum of light emitted from the center of the sun with that from its limb.¹⁵

William Huggins wrote to Stokes with his recommendations for insuring successful spectroscopic observations of the prominences during the upcoming eclipse.  Huggins urged placing a skilled observer in charge of an equatorially mounted telescope of 3-3/4 inch aperture capable of being moved by hand.  It was Huggins' opinion that even with a competent observer, the limitations of expense imposed on a Government eclipse expedition would make it difficult to obtain more than the "general character of the spectrum of a red prominence, that is, whether it be continuous, or consist of bright lines."¹⁶

Six months before the eclipse, in the February 1868 number of the Monthly Notices, Huggins disclosed, in his annual observatory report, that he had tried to view solar prominences without an eclipse.¹⁷  In this report, he made no mention of trying to make the red flames visible by filtering the sun's rays through a stack of colored glass plates.  Instead, he described a spectroscopic method drawn from his experience with nebular spectra.  Huggins reasoned that if prominences were gaseous, they would display an array of bright emission lines when viewed spectroscopically.  These colorful lines would suffer little dispersion in consequence of their passage through a prism, regardless of its power, and hence would barely show any diminution in brilliance.  Other light near the prominence, from the background sky, for example, or the solar limb, already known to produce a broad Fraunhofer spectrum could be significantly reduced in intensity by passing it through a prism of great dispersive power.  In this way, a prominence spectrum could be seen distinctly.

Huggins claimed to have been engaged in making such observations for the past two years.  While this was a bit of an exaggeration, it sufficed to establish him as a serious contender for priority in this limited arena.¹⁸  In fact, since, in early 1868, Lockyer was still awaiting the completion of his new and more powerful spectroscope, Huggins had an instrumental edge on him.  But Huggins' notebook entries show he was pre-occupied in 1867 with his experiments designed to measure the heat of celestial bodies, observing changes in the lunar crater Linné, and, by 1868, beginning his measures of stellar motion in the line of sight.  During this particular two year period, Huggins recorded very few solar observations and almost all of these were observations of sunspots.¹⁹

As the August 1868 eclipse approached, two British and two French teams equipped for spectroscopic, photographic, and visual observation were sent to India and Malaya.  Clouds obscured the beginning of totality at Jamkandi where one of the British teams headed by Lieut. John Herschel, son of astronomer Sir John F. W. Herschel, was waiting to observe spectroscopically any prominences that might be visible.  Fortunately, the clouds cleared sufficiently during totality for Herschel to glimpse the spectrum of a prominence.²⁰  The sight of the bright lines sparked an idea for a method of monitoring prominences on a regular basis, a method relying on intervening absorptive media very similar to that devised by Huggins nearly two years earlier.  Lieut. Herschel wrote to his father:

It has occurred to me that, since the whole light of the "flames" is of three refrangibilities only (or nearly so), dark glasses (could they be formed) which allowed these only to pass, would so enormously diminish the light from the Solar disk, as to enable the flames to be seen without the interposition of an opaque body, i.e. without an eclipse....  Is the idea a practical one?  If it is, these flames may come to be studied at leisure!²¹

The second British team, headed by Major John Tennant and stationed at Guntoor, was hindered by cloud cover during the second half of totality.  One team member, specially trained in advance by Warren De La Rue, took charge of obtaining a set of photographs during totality in continuation of De La Rue's efforts during the 1860 eclipse.  The team obtained six faint, but, in their opinion, satisfactory photographs of the solar atmosphere during totality which showed considerable detail in the structure of the visible prominences.  Tennant's spectroscopic observations of the prominences were thwarted by a recalcitrant clockdrive, although he noted seeing several bright lines in their spectra which he believed were coincident with some of the Fraunhofer lines in the solar spectrum.

Pierre Jules Janssen, France's pre-eminent solar observer, led his country's eclipse expedition to Guntoor.  The French team trained in advance to handle the mental and physical pressures of the work to be done during totality.²²  Before clouds obscured his view of the eclipsed sun, Janssen was able to subject two large solar prominences to spectroscopic examination.  The bright lines he observed led Janssen to conclude that the prominences were gaseous in nature.  He also surmised, when he saw how vividly bright the lines were, that they might remain visible under spectroscopic scrutiny even in full sunlight, just as Huggins had suggested independently several months earlier.  Janssen tested his hypothesis the next day. Remembering the approximate location of the larger prominences on the solar limb, he quickly detected their bright lines.²³

Meanwhile, Lockyer was still awaiting the completion of the new spectroscope being made for him by John Browning.  He had had, however, the opportunity to become acquainted with some of the eclipse observers' cabled reports.  Although Lockyer seems to have been ignorant of Janssen's discovery, he knew that others had confirmed the suspicion that the prominences produced bright line emission spectra.  The qualitative nature of these observations did not provide Lockyer with sufficient information to allow him to know which spectral lines to look for, but at least he was given hope that they would be visible.²⁴  On 19 October 1868, just days after obtaining his new spectroscope, Lockyer observed carefully the bright line spectrum of a prominence.²⁵  He sent out letters immediately describing his preliminary findings.  Because of the delay in receiving any word from Janssen regarding his own discovery back in August, Lockyer's announcement arrived at the Académie des Sciences in Paris at virtually the same time as Janssen's.  Lockyer and Janssen were awarded a special medal by the French government in honor of their independent discoveries.  No official recognition, however, was bestowed on either man by the British scientific societies.²⁶

Huggins did not see the bright spectral lines produced by a solar prominence until 19 December 1868, fully two months after Lockyer's success.  He failed in spite of the fact that, as we have seen, he had suggested a means of direct observation of the solar prominences without an eclipse back in February 1868.  He had the necessary instrumentation.  In fact, when he ultimately viewed the solar prominences for the first time, he tells us that he did so with his trusty spectroscope which boasted two dense 60° prisms by Hofmann and Ross:

Applied 2 prism spectroscope to edge of sun.  Saw at once the three bright lines.  The red very strong.  The F line like this [diagram] broader at the base.  The line near D apparently double, not certain of this, a little more refrangible than D.  Tried red glass, but could not see prominence.²⁷

Huggins' acquaintances were puzzled as to why he had been unable to see the spectral lines until then.  In November 1868, Warren De La Rue wrote to George Stokes:

It is curious that Huggins should have failed in discovering the "red flame" -- because he is a very skilful observer: -- had he been successful he would have anticipated Janssen and, by so doing, rendered great assistance to the Eclipse observers.²⁸

Agnes Clerke would later conclude that Huggins "devised various apparatus for bringing them into actual view; but not until he knew where to look did he succeed in seeing them."²⁹  This was a harsh verdict for an observer who, by the time Clerke handed it down, had achieved considerable respect for his care and perspicacity.³⁰

Huggins knew from his own experience that "fishing around the limb of the sun," as Lockyer so aptly put it, for elusive emission lines was a hit or miss activity.³¹  An individual who had seen the lines during an eclipse would be better prepared, but "not knowing what kind of lines would appear in the prominences,"³² or where to expect them, he remonstrated, "it would have been by accident only if I had succeeded in obtaining a view of the flames."³³  Huggins' insinuation that Lockyer had succeeded principally because he had some knowledge of the recent eclipse observations was not lost on Lockyer who angrily denied that he had been helped in any way by the findings of the 1868 eclipse observers.  Lockyer pointed to his 1866 Proceedings paper as evidence that he had been working toward just such a discovery on his own for two years.  John Browning and Balfour Stewart came to Lockyer's defense.  Huggins was unconvinced and unrepentant.³⁴

Huggins felt that once eclipse observers had definitively resolved the question of the character of prominence spectra and described where to look, actually seeing the spectral lines without an eclipse was merely a technical exercise, not a major discovery.  Clearly, Janssen had beaten everyone to the visual discovery.  But any skilled observer with the right information and sufficient instrumental power could have done that.  What really counted was figuring out that it could be done in the first place.  And on that score, Huggins believed he had beaten them all.  Lockyer may have had plans to examine the prominences spectroscopically in October 1866 -- but what were they?

Meadows has discussed the substance of this quarrel.  Further recounting of details here is unnecessary.  Suffice it to say that Lockyer continued to seethe long after the fact.³⁵  And Huggins, through his role in the dispute, became more aware of the need to establish and preserve his priority whenever he engaged in some research project he believed to be original.  Astronomical physics was evolving rapidly and one's priority required constant reassertion.  Investigators were provided with no clear boundaries or well-defined research guides.  What might seem peripheral at one time could prove to be central at another.  Thus, in February 1869, Huggins submitted a paper to the Royal Society's Proceedings describing his earlier thermometric work to preempt others' claims.³⁶  And, when Warren De La Rue published what he thought was a new method of observing the whole of a prominence without an eclipse by using only absorptive filtering, Huggins responded quickly, claiming he had already thought about that "three or four years since."³⁷

In summary, the development of a successful method of rendering the solar prominences visible to observers on any cloudless day had extremely important implications for solar observers.  Prominences could now be monitored on a regular basis just like sunspots without waiting for an eclipse, without begging for funds to sponsor an expedition, and without praying for clear skies during totality. Prominence activity could also be examined on the visible surface of the sun instead of just at the sun's limb.  When reports from all the 1868 eclipse observing stations had been gathered, there were hints that prominences were active solar features that could and did change in structure and appearance over very short periods of time. Near the close of the RAS meeting in January 1869, which was devoted almost entirely to discussion of the 1868 eclipse, Warren De La Rue observed:  "It comes to this, that the spots are the least frequent occurrences.  The changes of the faculae and prominences are much more numerous."³⁸

Astronomical physics itself was changing even more quickly.  It was becoming almost necessary for investigators, like the Red Queen, to run merely to stay in place.  The September 1869 number of the Astronomical Register opened with the statement, "The age is essentially a fast one....  [W]ith regard to our present subject [spectroscopic observations of the sun] ... the race for fresh discoveries is so evident and the competition among observers so keen."³⁹  The lack of rigidity in Huggins' research program served him well here.  His early attempts to view solar prominences without an eclipse, in spite of his lack of success, gave him the opportunity to play a central role in the ensuing priority dispute.  Instead of sitting on the sidelines as a nebular and stellar specialist, Huggins chose to jump into the fray with the solar observers.

NOTES
[click on footnote number to return to text]

• Preface

• Introduction

• Chapter 1—

• Chapter 2—

• Part 1—

The Astronomical Agenda:  1830-1870

• Part 2—

"A sudden impulse..."

• Part 3—

Reception of Spectrum Analysis Applied to the Stars

• Chapter 3—

• Part 1—

Cultivating Advantageous Alliances; Opportunism and Eclecticism

• Part 2—

Opportunism and Eclecticism (continued)

• Part 3—

Achieving "A mark of approval and confidence"

• Chapter 4—

• Part 1—

The Solitary Observer

• Part 2—

Celestial Photography

• Chapter 5—

• Chapter 6—

• Part 1—

The Red Flames

• Part 2—

The Eclipse Expedition to Oran

• Part 3—

Photographing the Corona Without an Eclipse

• Part 3—

The Bakerian Lecture

• Conclusion

• Note on Sources

• Published Papers of William Huggins

• Bibliography

• Vita