The sun. [volume] (New York [N.Y.]) 1833-1916, 1900-10-28
FUTURE GREAT TELESCOPES
now ARE THEY TO mr. CONSTRUCTED
AND N8HNTEDP
Limits. Indicated Rather By the Cost Of
installation Than by sire Tho Pari,
Telescope Of 1000 where Telescopes
ShnuIh liz PLACED TO lie Of Moat Iae.

The British Association for tho Advance
ment Of Science has lately instituted n separate
department OF astronomy, which. until this
year, was associated with physics.
Al the last meeting Of tho association an adj
dress was delivered before the newly formed
department Of astronomy by its presiding
o%f1cer. A, A. Common. himself nm amateur
observer Of ability, best known. however. az
the mak0r Of several large rejecting telescopes,
one Of these Is the three-foot reflector now at
the lick Observatory. which. In the hands OF
the late Prof Kceler. has done such brilliant
work In the photography Of nobulg, and an
other Il the f~ve-foot reflector mounted In MIR,
Commons private observatory In London.
where It's powers are somewhat handicapped
by the wretched climate.
MIR. Common has taken for the subject OF his
address the progress Of astronomy since the
year 1891, when the British Association was
founded. and devotes especial attention to the
development Of the telescope during the years.
a topic which he Is especially well fitted to
treat. 1t is proposed to give some account OF
this very interesting subject, taking Mr.. Conn
mons address rs n text, without following his
remarks with eXactnes-,
About the year 1880 there we're few observ-
atorles In England and none at all In America.
Sir William Herschel had made rejecting telc-
scopes Of great excellence and large size-his
largest had a mirror ~ four feet In diameter
and his extraordinary discoveries OF the
planet Uranus u7sl), rftwo aatellites to Uranus
u7s0, Of two satellites TO Saturn U7se) Of re-
volving double stars. Of Thousands Of Lebula.
had made the reflector famous The reaectors
OF those DAYS were provided with mirrors cast
from spectrum metal. The mirror got the
proper pdrdbolic shape by the very process
used to produce the necessary polish 10 lts re-
fectlng surface When the surface furnished,
ms IL was sure 10 do IN time. the mirror had to
be repolished and in the process its parabollc
! figure was lost and had 70 be again established
Thus. In effect. n new mirror had to be made
every time that An old mirror was repollshed
| and this reworking required several days. The
famous three-foot and ~six-foot mirrors Of Lord
Rose wero u spectrum metal and were sub
iect to these hard conditions. Modern re-
! Electors are made In q very different fashion.
They are formed out n blocks Of glass and then
| covered with a very hin coating OF quick
silver The glass 10 ground 10 shape once for
! all. When II Ia once figured II remains In shape
forever. Its coating ~ quicksilver can he
replaced whenever It becomes furnished. and
II IA a matter Of n few hours only 70 renew the
quicksilver film. The saving In time, and
especially In the labor Of the skilled optician
Is immense
In Herschel's day II was impossible 70 employ
the sIlver-on-glass process. Blocks Of glass
Of tho proper thickness. three feet in diameter
! or more. could not he obtained. II was NOT
until glass manufacture had reached great
| eycellence-ab iSJO-that the material for
| large rehectors could he obtained. and IL was
not until Liebig had discovered his method
Of silvering glass surfaces that the coating
III 4 ----1tal L. NItg RAaRnn
n. in many others. science waits upon tho arts,
! having in the first instance suggested the proc
esaes which the arts hive successfully followed.
| The difficulty in procuring optical glass in blocks
Of .uficient size hindered the de development Of
the refracting telescope also. In the year
1924 the largest rcfractinR telescope had an OBJ
lect glass about Ave inches in diameter. Ii
was with this instrument that Sir John Her
! schel. son Of Sir william, made his series OF
observations OF double stars. Ne tho an n
Rlassmaking was improved scielce felt the
benefits. In tho year 1924. also. n ntnc-inch
refractor was mounted AT the observatory Of
. porpat In Russia, a l5-inch was provided for
the Imperial Russian observatory at Pulkowa
in ls44 and soon afterward one of the same
1 size was mounted AZ the observatory Of Harvard
| College lsi1. Since that time large refractos
have followed In quick succession. and II Ia a
source Of pride 70 Americans 70 remember
' that the best Of them have come from American
workshops. The Chicago rcfractor with which
| the companion to Sirius was discovered las
an aperture Of eighteen and a half inches.
and IL was made by Alvan Clark In 1890. The
Princeton telescope Ia twenty-three inches
in aperture, that Of Washington is twenty-six
inches. the Clark telescope at Pulkowa is thirty
inches: the Lick telescope Is thirty-six inches:
that Of tho Yerkes observatory In Chicago.
IN, largest OF It's kind, Ia forty inches.
The question arises whether the limit has
been reached, and the answer must be that stNl
larger TELESCOPES will. no doubt, I've made, but
that we are certainly near TO the limit-and
this for various reasons, some Of which
will be considered below The great re-
necting telescopes q the world have
all been made In Europe. Only one
OF them Common. three-foot mirror
Is mounted in America. Paris and Melbourne
nave four-toot refectors; Mr. COMMON has a
f~ye-foot. Lord Roses great reflector now
little used has a s~ix-foot speculum Tele-
scopes Of the kinds iust mentioned are mounted
on stands Of complex construction. and to be
truly effective they must he covered by revolv-
ing domes The dome Of the Lick telescope Is
about seyenty-fve feet in diameter and cost
tome 5000 O The mounting for this telescope
cost pbout $4,000 The elevatiug platform
tsixtv feet in diameter dat rings the observer
10 the eye end II tho telescope cost another
$4,000. Before the object glass dtscif costing
$50, OOOl can be made e~1 ective complex nppa-
ratus. valued al about $4,000, must be pro-
yided. with every Increase Of size the cost Of
the subsidiary apparatus increases and II in
creases very rapidly. II the size Of a dome is
doubled It's cost Ia increased not twice but six
or eight times
One lesson energies directly from these con-
siderations. 1t is folly to place such expensive
Instruments az sites where the atmospheric
conditions are not favorable Mr. Common 5
thrco-foot reflector mounted. as II Is, on Mount
Hamilton, in a perfect atmosphere. Is inf
nitely more effective than It was in England,
The great refractor has done far more service
at the Lick observatory than II IL had been sit
uated at New York. Boston or Chicago. When
ever a great telescope Is built II 8 hould then be
placed at a station favored by atmospheric
conditions.
There are other ways OF mounting and using
large telescopes The function Of the equato-
rial mounting IL to cradle the observer 10 direct
his telescope to a star in the east. say. and 10
enable him TO follow It in is diurnnl course
toward the west-from rising toward setting
To thia and tho telpsgope tube Is driven nv a