The Irish standard., 1896-11-28
A Great Telescope.

The object glass for the Yerkes tele-
scope has been completed and tested, and
the giant instrument will soon be in ac-
tive use of the observatory established by
the University Of Chicago at Lake Gen
eva. wis. The Yerkes telescope now
takes precedence Of the Lick telescope, anu
an interesting comparison OF the TWO may
be made. The chief thing in telescope
is the object glass, which concentrates
the rays of light at focus, and THESE pro
duce an image. of the object looked at.
This image is nagnified by means OF an
eyepiece resembling microscope: Two
things %. are specially to be CONSIDERED
First, that the larger the object glass is
the more light it will bring to its focus;
and. second. that the more nearly per1ec1
the object glass is the more nearly 1ault-
less the image at it's focus VIII oe' :Wlth
more light one can see fainter objects;
with a better image one can use higher
n,a ,agnifying power. The size Of all OBJ
iect glass is expressed by it's dirt1ameter
in inches. The Lick telescope has an OBJ
iect glass 36 inches in diameter, While
the diameter of the object glass OF the
Yerkes telescope is 40 inches. In estlmat-
ing the comparative powers OF the two
glasses for gathering light at the focus
we must square their numbers. Inc
square of So is 1885, and the square OF 4O
Is I 900: so we see that the light IN the
yerkes telescope is to that IN the Lick
telescope about as 16 to 13. They may
he compared in another way: ii we dlsre-
gard the small loss that the rays OF llgnt
suffer. in passing through: the glass; we
nav say that the Lick telescope shows
@ stars So faint that more than 10000 OF
them would be required to equal In
brightness single one Of the very faint
est stars the naked eye can perceive,
while about 10000 of t. faintest stars
that the new Yerkes telescope can reveal
would he required for the same purpose.
When we consider that, broadly speaking
tho fainter stars are at a greater distance
that the: brighter ones, 1t becomes evil
dent that the new glass VIII enaole as
rronomers to penetrate much tarther into
the mysterious and wonderful depths OF
snace than they have been. aDle 10 oo.
popular. way OF estimating the power OF
a telescone is to calculate the apparent
distance of the moon when seen through
It with it's highest 1nagnlfylng power
Under theoretically perfect conolnons
good telescope should bear Inagnliyihs5
nower of 100 diameters for each inch OF
anerture. In the Lick telescope this
would mean power of $500 dianeters
But in: practice the highest power gc us used ed
with that OF the telescope is $500, which
brings S.t the moon within an apparent dis
tance Of about 80 miles: Supposing that
a correspondingly high power is used
with the Yerkes telescope, it Will snow ths
moon at an apparent distance OF -abou;
30 miles In the actual study OF the moon
however, such highspowers are. very rare
" employed, because, owing to atmos-
phereic disturbances and other sources
of imperfection, the image is not distinct
enough in its minor details to- bear ex-
treme magnification. Even with the larg-
est telescopes the moon is best seen when
brought within an apparent distance OF not
less than 800 miles But if the object
glass is equally good and the air equally
Steady, at the same apparent distance tbs
Yerkes telescope will show finer details
on the moon than the Lick telescope can
show.