http://www.skyandtelescope.com.au 23nebulosity around the Pleiades. Barnard had taken
wide, four- and ten-hour exposures of the Pleiades
and their neighbourhood. These captured not only
the cluster’s internal wisps but dim, diffuse arcs
extending out. At the April 1896 meeting of the British
Astronomical Association, Roberts argued that the
arcs were merely “circles of halation” caused by defects
or reflections in Barnard’s lens. Furthermore, they
did not appear in his own exposures using a 125-mm
wide-field camera that he had recently installed.
A testy Barnard dashed off a note to the Royal
Astronomical Society, suggesting that the “previous
experience of Dr. Roberts with the diffused and faint
nebulosity about 15 Monocerotis... might warrant
some hesitation on his part in denying the existence of
a similar region about the Pleiades.”
Barnard followed up on October 22, 1898, with an
announcement to the Royal Astronomical Society that
Herbert C. Wilson at Carleton College in Northfield,
Minnesota, and Solon I. Bailey at Harvard had both
recorded images of the supposedly fictitious nebulosity
around the Pleiades. Barnard didn’t mention Roberts
by name, but it’s clear where his invective was
directed: “These nebulosities... have been amply
verified (if such a verification were at all necessary)...
It would therefore appear that a failure to show these
remarkable features with an ordinary portrait lens
and an exposure [well short of ] 4 or 5 hours must be
attributed to something else than their non-existence.”
By now the weight of opinion favoured Barnard.
The Royal Astronomical Society had published more
than two dozen of his papers and awarded him its Gold
Medal. Two other trailblazers of deep sky photography,AndrewCommonandDavidGill,openlydisputed
Roberts’ideasaboutnebularimaging.An1898review
by astronomer R. A. Gregory, at Queens College,
London,lavishedpraiseonthewide-fieldcameraas
an astronomical instrument. In what he termed the
“warm controversy” between Barnard and Roberts,
Gregory fully endorsed the reality of the nebulous
clouds on Barnard’s plates.
Several years passed. The final parry between
RobertsandBarnardoccurredin1903inthepages
of theAstrophysical Journal.Roberts had submitted
an article on William Herschel’s visual catalogue of
faint,extendednebulosity.Overtheprevioussixyears,
Robertshadphotographedall52ofHerschel’sfaint
nebular regions with both his 50-cm reflector and
wide-field camera, yet found nebulosity in only four.
Evidently, one of history’s keenest visual observers had
imagined all the rest.
Soextraordinarywasthisclaimthatthejournal’s
editorsengagedBarnard,nowatYerkesObservatory,
to write a rebuttal. The articles appeared together.
BarnardpointedoutthatRoberts’sexposure
timeof90minuteswastooshorttoshowveryfaint
interstellar clouds; his own camera shutter typically
remained open for four or five hours. As one might
tutor a neophyte, he reiterated the primacy of surface
brightnessfornebularvisibility.Toillustratethepoint,
he described a picture he had taken of Herschel’s
Region 27 in Orion through an ordinary lantern-slide
projectorlensamere40mmacross:“Mostofthegreat
curved nebula is clearly shown,” he wrote of the huge,
ghostly arc now known as Barnard’s Loop, “especially
the region described by Herschel... There is thereforePOWER PAIRS
Left: Barnard, in
his “Esquimaux
coat,” guides on
a star as he peers
through Lick
Observatory’s
165-mm Clark
refractor and
adjusts its fine-
motion wheels.
Riding on the
refractor is the
wide-field Willard
camera with its
15-cm portrait
lens. Barnard
sometimes guided
an exposure for
five hours at a
stretch. Right:
With his 50-cm
reflector and 175-
mm refractor on
a dual equatorial
mount, Isaac
Roberts ran one of
the finest private
observatories in
England. But the
large image scale
of the big scope
made it ineffective
for large, diffuse
nebulae with
low surface
brightnesses.ISAAC ROBERTSA SELECTION OF PHOTOGRAPHS OF STARS, STAR-CLUSTERS AND NEBULAELICK OBSERVATORY / UC SANTA CRUZ