New-York tribune., 1900-09-23
ADVANCE IN ASTRONOMY.
VISUAL TELESCOPES USED PHOTO
GRAPHICALLY
SIMPLE MEANS WHEREBY GREAT ECONOMIES
MAY BE EFFECTED.

most extensively in the last few years are in
the application of photography and spectroscopv
20 their researches The advantages of photo
graphing celestial objects, like star clusters
sunspots, nebulR and lunar scenery are enor-
nous Pictures are thus obtained which are
vastly superior to drawings and they are made
In DIR less time The record is much more ac-
oirate, too. Stars and nebula whose existence
mas been unsuspeCted are discovered in this
manner Finally, by multiplying the prints
from a Photographic plate. astronomer can
give another chance to review and criticise his
work and conclusions
But the telescope which is designed simply for
visual work in an observatory is not adapted
10 Photography The reason is simple. The
object glass Of this instrument acts to a certain
extent like prism In order to bring the rays
TO a focus ii must bend them from their orig
inal paths But a lens. like prism, will bend
light rays of one color more than those of an
other. Hence, the focus for the violet and blue
Is not the same the focus for the yellow; |
and none Of the three correspond in position to
the red ray focus To some extent this peculi- !
arity is overcome by making the two lenses
which compose objective out of different
materials One is crown glass and the other
flint One has a different index of refrac-
ton" from the other. and hence may be used to
correct the optical properties of the other.
Nevertheless, even when telescope is com !
pleted in accordance with the makers design, |
this fault viii remain TO certain extent and
the correction is applied that the eyepiece
may be adjusted to the focus of the yellow rays.
This is the most desirable plan when purely |
ocular work is to be performed 1t will then
be found that the red rays reach focus sew |
eral inches up inside the tube. and the blue
rays meet several inches outside the visual |
focus Now, photography is accomplished main |
ly with blue and violet rays. Hence, if a photo
graphic plate is placed in this yellow ray focus
the image formed by the blue rays will not be
clear and distinct, but badly blurred
With small instruments and on exceptional
occasions astronomers who have had a special |
photographic training like Mr.. Barnard. will rig
a pIateholder at the right distance from the |
eyepiece and obtain good results. But II re-
quires great skill to guess where the blue ray
focus is. Indeed, where anything like system |
attic photography is contemplated it is Gus
tomary to rely upon one of two eXpedlents.
Sometimes telescope is built exclusively for
this line of work The objective is corrected so
that the actinic violet rays concentrate at just
the right point But such instrument is |
worthless for visual work. and 1t is employed |
only for photography This is the case, for in |
stance with the fine 4-inch glass given to Har- |
ward by the late Miss Caroline w. Bruce The
other resource is to provide an additional correct
ing lens that can be swung into position in front
the Lick and Greenwich obserVatOries. q 1t |
works admirably, but il has drawbacks The |
corrector is heavy and costly. 1t absorbs
good deal of light and reduces the Photographic |
mower Of the instrument accordingly; and the |
focus is shortened, so that the platehOlder must
be thrust up inside this tube little distance
from the eye end
A new way of meeting this difficulty is re
nOrted from the Yerkes Observatory, which,
though conducted under the auspices of the
University of Chicago. is situated at Williams
Raw Wis In the September number of The |
Astro-Physcal Journal Professor George f.
Hale the director Tecounts some recent expert |
meuts by c. w. Ritchey, of his staff the
great 4O-inch telescope at this observatory was
designed for visual work and has no correcting
Ions But such an attachment is now found to
he unnecessary If a thin yellow screen is placed
in the focal plane of the instrument and pho-
toqraphic plate in immediate contact Herewith
the most gratifying results are secured. The
mixtures show a sharp definition never before
exhibited in work done with visual telescopes !
Snecial screens have been made under Mr.
Ritchey's super yision. They are of plate glass,
coated with colIodion. having delicate, green
ish yellow tint
Professor Hale says that Mr.. Rtchey sug- |
a,ested this expedient eight years ago, and tried
if on the rnoon-a very bright object-in 1897. |
LOO E /