The Providence news. (Providence [R.I.]) 1891-1906, 1900-12-20
BIG PICTURE
OF The MOON.
Photograph Was Taken Through
yerkes Telescope.
new COLOR SCREEN DEVICE.
Views Of Star Clusters Seen Through tho
Furiy-Inch Lens Reveal Thousands Of
Separate Bodies
new COLOR SCREEN DEVICE.

G. w. Ritchey. superintendent of the
Instrument making department OF the
Yerkes observatory, has recently solved
a problem which has been "A source OF
difficulty 10 astronomers, and by means
OF a simple and inexpensive device has
made an Improvement OF much import
ance I'm astronomical photography.
With the possible exception of the new
horizontal telescope at the Paris Exposi-
ton, the Yerke's telescope SO ft.long, with
lens 40 inches I'm diameter. Is the most
powerful refracting telescope in the
world.
The views OF celestial objects given by
this telescope are extremely fine. and the
measurements obtained with IL are pro-
portionally accurate.
IL has not heretofore been available,
however. for obtaining direct photo
graphs OF the heavenly bodies. for the
reason that IL is a visual telescope. and,
as Is well known, the lens of telescope
intended for visual purposes Is shaped
very differently frorA that of telescope
intended for photography.
SCREEN OF YELLOW GLASS.
TO explain the difference very briefly.
the lens of visual telescope Is ground
and polished TO such a shape that II
unites I'm one sharp focus those rays OF
light which most strongly affect the eye,
namely. the yellow and green rays: while
the lens of a Photographic telescope is OF
such form that IL unites at Its focus the
blue and violet rays, which most power
fully affect the Photographic plate.
IL is at present impossible 10 make
large lens. OF simple form, which will
unite rays OF all colors I'm one sharp focus,
thus combining the properties OF both
visual and Photographic lens.
But by using Photographic plates of a
kind which are extremely sensitive to
yellow light and by placing over these
plates an optically perfect screen OF yell
low glass, which allows only the sharp
yellow or visual. image produced by the
great lens To reach the sensitive plate,
Mr.. Ritchey has succeeded In utilizing
the ic-inch visual telescope for photo
graphy OF celestial objects.
The resulting photographs are extreme
ly sharp and distinct. and on account OF
the great size of the telescope used these
negatives show minute details which il
has never before been possible to photo
graph.
GLoBULAR STAR CLUSTERS.
The results demonstrate the available
ity OF all visual telescopes for astronoml-
cal photography by using very inexpen-
sve means
Perhaps the severest test which has
been made of the new methods Is I'm
photographing the dense globular star
clusters In which thousands OF minute
stars are massed so closely together that
only the most powerful telescope can
show them separately.
These clusters are so far from us that
most of them are entirely invisible to the
unaided eve.
MIR. Ritcheys photograph OF the cluster
Messier ~, I'm the constellation OF Her
cules shows 5000 stars perfectly separated
and distinct in an area OF the sky only
ten minutes OF arc square.
In his Photograph OF the cluster Messier
15 which is much more compressed than
Messier i3, more than 1000 stars are
shown.