The Savannah morning news. (Savannah, Ga.) 1900-current, 1902-04-20

oratory into astronomy on a large
scale, and will enable the astronomer
to apply to the details of his work re-
cording and measuring instruments
which cannot be operated with any
telescope that must be balanced and
moved by an astronomical clock.
Photography in Astronomy.
We are accustomed to think of a
telescope as a long steel tube equipped
with lenses for bringing close to the
observer the image of the star or neb-
ula- We think of the astronomer as
man doomed to spend the nights of his
life in cold dome, open to the wealth
er, while with the eye glued to the
smaller end of the big instrument he
moves hour after hour across the floor
of the observatory, slowly noting down
the phenomena which he there OBJ
serves. These phenomena the layman
must take on the astronomers word.
In the last twenty years, to be sure.
photography has come into vogue DIS a
method for recording these observa-
tons. More recently it has been dis
covered that astronomical photographs
may show things which cannot be seen
with the naked eye even through the
most powerful telescope. The atrono-
mer has learned that by attaching a
BlAObOSA pldlG W lilS lClGStODG anu
keeping it at a given point in the heav-
, ems for several hours he will find on
developing the plate that he has bet
ter image of the stars in that part than
his eye gave him-since his eye was
subject OF fatigue and the plate was
not. Besides, he will have a record
that he may show wherever he will
and which is as plain to any one with
out a telescope as it is to him. More
over, he may find-as has frequently
happened-that he has countless stars
and perhaps even nebulae recorded
which he has never seen. Though he
may gaze or hours he cannot see these
with his eye. Yet by the use of Sue
cessive plates he can study their phe-
nomena from night to night and pre-
serve a record of them that will show
whether any change in position or
shape is taking place.
Taking these stellar photographs is
a difficult job. In the first place, the
lenses of the telescopes are made for
the use of the eye. Glass will not re-
tract all rays of light equally. Conse-
quenty, when we wish to focus rays
with a lens we must use one which
will bring to a focus the particular
rays we need to use.
Our eyes see with the yellow and
green rays, and these are the ones that
are focused IN visual lenses, such as
are used in telescopes. Ordinarily the
Photographic plate is sensitive to blue
and violet rays, and to those ultra vio-
let rays which are beyond the eye.
When il was decided to use the big
Lick telescope for photographing the
heavens it was found necessary to
construct an additional lens, at the
cost of $4,000, to put on over the Wis
ual lens, that it might focus the "ac-
tinic" rays needed. That shortened the
focus of the telescope and made it
awkward to handle. But it enabled the
astronomers TO get very wonderful pic
lures, especially of the moon.
But Photographic plates need not be
sensitive to the blue and violet alone.
It is possible to "orthrochromatize
the plate-that is, to dye the solution
with which it is sensitized so. that it
will be affected by other rays. And in
this way plate can be made that
will be affected by the yellow and
green, though it viii still be more
sensitive to the blue and violet. By
introducing a yellow screen in front Of
the plate, so as to let no blue rays
through this plate can be used with
visual" lens, and the picture that
the eye sees can be thus recorded
that is, the image can be printed in by
the same colored rays that the eye
sees with. This is the actual process
employed at several observatories to-
day-notably at the Yerkes ObserVa-
tory, whore IL has made the 40-inch re-
tractor, the largest I'm existence a pho
tographic instrument.
But when they came TO take photo
graphs with this big telescope AZ the
University Of Chicago Observatory the