844 
that the photographie and optical resolving 
power may be equal, it is doubtful if this 
nebula has ever been photographed with an 
entirely suitable instrument. As it is a 
small object and photographically bright, it 
could be advantageously photographed with 
a reflector of unusually long focus. The 
lenses of refractors absorb the actinic rays 
to such an extent that for such instruments 
exposures of even twenty hours have been 
given to this nebula. With the Crossley 
reflector, under the finest conditions, on 
July 14th of this year, an exposure of 30 
seconds produced an image which was 
barely visible ; one minute a faint image ; 
two minutes a distinct image, and ten min- 
utes exposure gave the best general picture 
of the nebula. With one and two hours 
the plate was much overexposed. The focal 
length of this telescope is 174 feet, but if 
it were four times greater a far better photo- 
graph could doubtless be obtained, the neces- 
sary exposure then being about three hours. 
The ratio of aperture to focal length could, 
however, be reduced by cutting down the 
aperture, thus diminishing the aberration 
and atmospheric disturbances. But for an 
object like this nebula the aberration is in- 
sensible, the star images being excellent at 
half an inch from the axis; moreover the 
photographs were taken on nearly perfect 
nights when the definition would not have 
been improved by reducing the aperture. 
These photographs of the ring nebula 
show features described by observers with 
powerful visual and photographic telescopes, 
and others which appear to be new. The 
outline of the nebula is oval rather than 
elliptical, with faint structureless fringes of 
nebulosity projecting on both sides of the 
oval. .The ring has quite a complicated 
structure, as if made up of several narrower 
bright rings, interlacing somewhat irregu- 
larly, the intervening space being filled with 
fainter nebulosity. 
A comparison was given of the dimensions 
SCIENCE. 
[N. S. Vou. X. No. 25 
of the nebula as measured by Barnard with 
the Lick refractor, and as determined by 
Stratonoff on photographs obtained in 10 
hours exposure with a refractor of 33 cm. 
aperture, and as measured on a Crossley plate 
with 10 minutes exposure. The author’s 
measures give a somewhat greater size than 
the visual measures, and also average slightly 
more than on the Russian photograph. 
Lord Rosse’s drawing, published in the 
Philosophical Transactions for 1844, showing 
the interior space of the nebula to be crossed 
by a series of dark and bright bands, and 
hitherto generally deemed fanciful, is now 
confirmed, it is believed, for the first time. 
The actinic power of the central star, 
noted by many observers, is also confirmed 
by these plates, being faintly visible on the 
plate exposed for thirty seconds. It is sug- 
gested that the photographic strength of this 
and other central stars may be due to 
bright lines, probably of hydrogen, in the 
upper spectrum ; and the author does not 
anticipate difficulties in photographing their 
spectra. On all the photographs the central 
star is as clearly defined as are other stars 
outside the nebula ; there is no evidence of 
blending into the nebulous background. 
This is also the appearance of the star as 
seen with the 36-inch refractor. (Published 
in the Astrophysical Journal. ) 
W. W. Camppetit: The Wave-length of the 
Green Coronal Line. 
One of the problems undertaken by the 
expedition sent from the Lick Observatory 
to observe the Indian eclipse of January 22, 
1898, was the determination of the rotation 
of the corona from the displacement of the 
green coronal line. A powerful train of 
prisms loaned by Professor Young furnished 
the necessary high dispersion. A success- 
ful plate was obtained during the eclipse, 
and while measuring this in January, 1899, 
I learned that Lockyer had assigned a new 
wave-length to the green coronal line. On 
