Sky and Telescope - June 2016 - 39

ISAAC ROBERTS, A SELECTION OF PHOTOGRAPHS OF STARS, STAR-CLUSTERS AND NEBULAE

LICK OBSERVATORY / UC SANTA CRUZ

POWER PAIRS Left: Barnard, in his "Esquimaux coat," guides on a star as he peers through Lick Observatory's 6½-inch Clark refractor and adjusts
its ﬁne-motion wheels. Riding on the refractor is the wide-ﬁeld Willard camera with its 6-inch portrait lens. Barnard sometimes guided an exposure
for ﬁve hours at a stretch. Right: With his 20-inch reﬂector and 7-inch 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 ineﬀ ective for large, diﬀuse nebulae with low surface brightnesses.

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 5-inch 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 diﬀ used 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 favored 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, Andrew
Common and David Gill, openly disputed Roberts's ideas
about nebular imaging. An 1898 review by astronomer R.
A. Gregory, at Queens College, London, lavished praise on
the wide-ﬁeld camera as 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 Roberts and Barnard occurred in 1903 in the pages of the
Astrophysical Journal. Roberts had submitted an article
on William Herschel's visual catalog of faint, extended
nebulosity. Over the previous six years, Roberts had photographed all 52 of Herschel's faint nebular regions with
both his 20-inch reﬂector and wide-ﬁeld camera, yet
found nebulosity in only four. Evidently, one of history's
keenest visual observers had imagined all the rest.
So extraordinary was this claim that the journal's
editors engaged Barnard, now at Yerkes Observatory, to
write a rebuttal. The articles appeared together.
Barnard pointed out that Roberts's exposure time of
90 minutes was too short to show very faint 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 brightness for nebular
visibility. To illustrate the point, he described a picture
he had taken of Herschel's Region 27 in Orion through
an ordinary lantern-slide projector lens a mere 1.6
inches across: "Most of the great curved nebula is clearly
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