Drawing in the dark at Birr Castle

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Exactly what sort of illumination did Lord Rosse use, to make his exquisite drawings of the "nebulae?"

The crucial point is that, in order to see a galaxy through the eyepiece of Lord Rosse's giant telescope (whose speculum mirror weighed over 3 tons), the eye of the observer first has to be "dark adapted." In simple terms, the retina contains two types of receptors: rods and cones.

There are approximately 120 million rods compared with 6-7 million cones. Our daylight vision (cone vision) readily adapts to changing light levels in a few seconds, as occurs when walking indoors out of sunlight. The rods, far more numerous, are highly efficient receptors to photons of light - more than 1000 times as sensitive as the cones - but the rod adaptation process is much slower. It is the rods of the retina which facilitate our dark-adapted or scotopic vision, enabling an observer to see intricate but faint details at the eyepiece. After a optimum period of about 30 minutes, the eye becomes adapted to the dark.

Now comes the dilemma. Lord Rosse produced some of the most striking drawings of the Whirlpool Galaxy and of nebulae. How did Lord Rosse actually "draw in the dark" and retain his dark adapted vision? Were he to use any bright (white) source of illumination on his drawing paper, such illumination would almost immediately have destroyed his dark adapted vision.

"Drawing in the dark" could never have relied on light from the Moon, since it is only in the absence of a bright Moon that faint features could be visually discerned through the eyepieces of Lord Rosse.

Dark adapted eyes are best preserved in the presence of only a certain kind of light: photons of long wavelength (red light). Rods respond little to red light. Sir John Herschel knew this well. It is most intriguing to have learnt from world renowned historian Professor Owen Gingerich at Harvard University, that the great astronomer Johannes Kepler himself, in his Astronomía Nova, mentions how he had to use a glowing coal to read the scales on his sextant because it was too windy to use a candle. Glowing coals would best preserve dark adapted eyes!

In the translation of the Astronomia Nova by Donahue, Kepler writes:

I think we are 10 minutes too high. For the wind was blowing so hard that it was only by a glowing coal that we could cast light upon the scale so as to read it.

Professor Gingerich further shared his thoughts with us:

I think if you look at the magnificent drawing of Eta Argus in Herschel's Cape Observations, or the details in Lord Rosse's drawing of M51, there is no way they could remember such intricate detail to recreate it later. Therefore it is entirely plausible that they used very dim lanterns, because when dark adapted not much light would be required, and they probably polished up the rough but detailed sketch the following morning.

Some further clues were kindly provided to us by Richard Handy in the United States. He drew our attention to Reminiscences and Letters of Sir Robert Ball, edited by W. Valentine Ball in 1915. Those reminiscences describe the period when Sir Robert Ball actually worked at Birr Castle, Parsonstown, in the period 1865-67. Sir Robert Ball had two principal duties: to tutor the three younger sons of Lord Rosse and to serve as an astronomer there.

Sir Robert Ball describes tutoring the three younger Rosse sons, as follows:

When I went to Parsonstown, in 1865, Lord Rosse was advanced in years. He no longer took an active part in the work of observation, but he evinced a lively interest in all. Lord Oxmantown, Lord Rosse's eldest son, was not one of my pupils. They were his three younger brothers, who are now the Hon. and Rev Randal Parsons, the Hon. R. C. Parsons - a well-known engineer - while the youngest is the Hon. Sir C. A. Parsons. It has always been a great satisfaction to me to remember that I had the great honour of instilling the elements of algebra and Euclid into the mind of the famous man who has revolutionized the use of steam by his invention of the steam turbine. It would seem that he inherited his father's brilliant mechanical genius, with an enormous increase in its effect on the world.

Robert Ball poignantly describes his observing at Birr Castle, thus:

Let me describe the scenes and conditions amongst which my life for the next two interesting years was to be passed. The residence of the Earl of Rosse is at Birr Castle, in King's County, about eighty miles from Dublin. Birr Castle is situated at the little town, which was then officially known as "Parsonstown," but to the inhabitants as "Birr." Quite recently I believe the official designation has been abandoned, and the Post Office only recognizes "Birr."Birr Castle is a noble building of modern erection, surrounded by a moat. It is situated in a beautiful park, through which two pretty rivers flow, and these unite in a single stream before they leave. The park has also a large artificial lake, ingeniously constructed by Lord Rosse himself, which is the perennial home of innumerable wild duck. Several instances of Lord Rosse's consummate mechanical skill are to be found about the grounds. Visitors used to stand gazing in wonder on a water-wheel which, being turned by the waters from the lake, raised water from a drainage system connected with low-lying lands around. A suspension bridge was thrown across the river close to the castle. The outstanding feature of Birr Castle, by which it will be forever famous in the annals of science, is the mighty telescope. Between the lake and the castle are two great walls, which are now somewhat overgrown with ivy. I have been told that Visitors entering the gates of the park for the first time have driven up to these walls in the belief that they were approaching the castle itself, which is not visible from the park gates. Between these two walls there swings a tube sixty feet long and six feet in diameter - a tube large enough to be the funnel of a good size steamship. At the lower end of this tube is the mighty mirror or speculum. Lord Rosse's telescope is what is known as a "reflecting telescope" - a reflecting telescope of the Newtonian type. The instrument is raised by means of a winch, which is place towards the north, and the observers who are to use the telescope have to make their way to the galleries. It is characteristic of this type of telescope that the eye-piece is at the top of the tube, not, as in the refracting instruments; at the bottom four men had to be summoned to assist the observer. One stood at the winch to raise or lower, another at the lower end of the instrument to give it an eastward or westward motion as directed by the astronomer, while the third had to be ready to move the gallery in and out, in order to keep the observer conveniently placed with regard to the eye-piece. It was the duty of the fourth to look after the lamps and attend to minor matters.

Sir Robert Ball continues:

On fine evenings I would go to the observatory as soon as it was dark. The observatory proper was a little building containing two small instruments, close under the shadow of the two great instruments outside. One of these was the great reflector already mentioned. The other was the "three-foot instrument" that is to say, an instrument having a mirror three feet in diameter, the tube of the telescope being ten times as long as the width of the mirror. The great six-foot instrument was, however the one which we employed for important observations. I shall suppose that we are ready to commence a night's work. The assistants above referred to are already at their posts. Up we climb to the lofty gallery, taking with us a chronometer, our observing book, various eye-pieces, and a lamp. The "working list" as it is called, contains a list of the nebulae, which we want to observe. A glance at the book and at the chronometer shows which of these is coming into the best position at the time. The necessary instructions are immediately given to the attendants. The observer, standing at the eye-piece, awaits the appointed moment, and the object comes before him. He carefully scrutinizes it to see whether the great telescope can reveal anything which was not discovered by instruments of inferior capacity. A hasty sketch is made in order to record the distinctive features as accurately as possible.

It was clear that the "lofty gallery" could accommodate heavy items of equipment. We read in the following section (also from Sir Robert Ball) that ...

Lord Oxmantown was also an assiduous observer. Many a night did we spend together at the great telescope. Astronomy was just then beginning to quicken with new life under the great impulse that had been given to it by recent spectroscopic discoveries. A spectroscope (then regarded as ofcolossal dimensions, for it weighed about seventy pounds, though that itself would be nothing in comparison with the spectroscopes now used at the Yerkes Observatory) had been built from

Lord Rosse's design. By means of it we saw that superb spectacle, certain lines in From Seeds to Stars the spectrum which announced the gaseous character of the Nebula in Orion. 69

With infinite patience Lord Rosse devoted years to making a drawing of the Great Nebula.

We wanted to know more about the kind of lantern that the Birr Castle observers used to preserve their dark vision. Their patience was astounding; the drawing of the Orion Nebula (Figure 35) was based on almost twenty years of observation. We were on a detective trail to actually locate such a lantern, with absolutely no leads as to the details of such lamps or lanterns, until David contacted Andrew Stephens in the United Kingdom. Andrew Stephens has a great interest in the history of astronomy, and has a large collection ofantiquarian books, including drawings by Charles Messier. Moreover, Mr Stephens has close ties with the present Lord and Lady Rosse at Birr Castle - and herein comes a most extraordinary discovery.

Could it be true that the lamps used by Lord Rosse actually had a brownish-yellow glass in front of them, so as to allow intricate details seen in the eyepiece to be transcribed to paper by Lord Rosse and his assistants?

By a lucky coincidence, the present Lord and Lady Rosse had recently cleared the attic space above a workshop tower used for scientific experiments, and the photographic dark room. Among the objects discovered here was a mysterious lantern ...

The answer: Item found!

Lady Rosse communicated to Andrew Stephens that she had located a lantern (Figure 36), with a yellow brown glass! She writes: "John Weafer found it when we started to get into the attic area at the top of the tower."

What is most extraordinary about the discovery is that the lamp or lantern does not have transparent glass in front of it, but rather a brown-yellowish glass. A lantern with a filter, to Shrouds of the Night preserve dark adapted eyes, while at the same time allowing some of the finest astronomical

70 drawings ever made, to emerge from Birr Castle!

Andrew Stephens writes:
Figure 36 [409]

Looking at the lamp we can deduce it sat over an oil burner which would have fitted in the bottom, this being a vessel containing an oil, probably whale oil, and a small wick casting a low intensity, reddish light, sufficient to illuminate a page of drawing paper, but not so bright as to destroy the sensitivity of a dark adapted eye to any great extent ... we can also guess there would have been some sort of "funnel" fitting over the metal rim visible at the top. We might guess this would have been made of glass or metal and when in place would have ensured the wick burned evenly and without flickering too much even in a light wind.

The most extensive all-sky surveys before the photographic era were conducted by father and son Sir William Herschel and Sir John Herschel, from about 1780 to 1860. The detec-Shrouds of the Night tor was the eye. South Africa features prominently in the story, for it was from there that the

72 monumental survey of the southern skies was undertaken.

Sir John Herschel departed from Portsmouth on board a wooden (teak) sailing ship, the Mount Stewart Elphinstone - belonging to the Dutch East India Company - on 13 November 1833, arriving safely at the Cape of Good Hope, South Africa on 15 January 1834.

The explorer William John Burchell (1781-1863) in his Travels in the Interior of Southern Africa poignantly describes the view of land from sea, as one nears the Cape of Good Hope. He writes:

At five in the afternoon, (13th November, 1810) the sailors on deck, who had for some time been anxiously looking out, called to us that land was in sight. At this pleasing intelligence we hastened up from the cabin, and although nothing could be seen but a small cloud, which seemed fixed on the horizon, and was at first not very easily to be distinguished, the captain, who was well acquainted with the singular appearance of the cloud which rests on the Table Mountain during a south-east gale, declared that the land which we had now before us was that of the Cape of Good Hope.

It appeared gradually and slowly rising out of the ocean, while our sails, well fitted with the gentle gale, bore the gliding vessel of the deep ... Every other thought was banished, and our whole attention was now turned towards the distant cloud ... every mile we advanced added some agreeable idea to the animating anticipation of my feelings on first setting foot on the land of Africa ... Table Mountain and Lion's Head were very easy to be recognized by the peculiarity of their form ... the remarkable cloud which covered the top of Table Mountain, resting upon it with all the appearance of a ponderous substance ... Its thin misty skirts no sooner rolled over the edge of the precipice, than they rarefied into air and vanished . The western part of Table Mountain, with its rocky precipitous side cleft in deep ravines, rose majestically out of the ocean ...

As we advanced nearer the shore, the mountains displayed an imposing grandeur, which mocked the littleness of human works: buildings were but white specks; too small to add a feature to the scene; too insignificant either to adorn or to disturb the magnificence of nature.

Table Mountain was well known to early explorers (Figure 37) and astronomers (Figure 38). A photograph secured in ca. 1880s of the grand Table Mountain is reproduced in Figure 39.

From naturalists and botanists such as William Burchell (after whom Burchell's rhinoceros - the famous white rhino - and Burchell's zebra equus burchelli are both named) to astronomers such as Sir John Herschel, the Cape was to open up multitudes of treasures, from the Earth spawning its flora and fauna below, to the starry skies above.

Writes N.S. Dodge of Sir John Herschel's anticipation to explore the southern skies, whilst on board the Mount Stewart Elphinstone:

No one knew so well as the great astronomer of whom we write, even before, while recumbent on the deck of the vessel that was bearing him through the tropic zone, he watched for hours together the shifting panorama of the star-fretted vault, how the moon appeared brighter, fairer, and better defined through a more transparent atmosphere; how the planets seemed to be other orbs; how the stars, long watched in a northern sky, drooped toward the horizon, and were at length looked for in vain; how orbs, which, to his former vision, had modestly moved along the southern outskirts of visible creation, now marched majestically overhead, each "Walking the heavens like a thing of life," while new and strange bodies ascended high and higher, until the old earth had passed away and a new heaven was aloft; nor how the Via Lactea, in the neighborhood of the Centaur and the Cross, coupled with profuse collections of nebulae and asteroids, stars and constellations, makes the southern sky the most magnificent star-view from any part of earth. Like the sources of the Nile to the untraveledgeographer, or the ice-cliffs of Greenland to the student of arctic voyages, he knew well what a personal inspection would place before him, and though the civilized world rang with applause at his sacrifice of home and its comforts, and country and its honors, for the sake of science, yet true philosophers knew that the compensation, present and future, Shrouds of the Night far outweighed the loss.

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