The Portland daily press., 1889-05-25
A NEW Colossal TELESCOPE.
Its Dimensions, Comparative Power
and Line Of Future Research.

Another large telescope has recently been
completed which bids fair to rival even the
one at the Lick observatory in power. 1t is
the property Of Mr. A. A. Common, who re-
sides at Ealing, England, where the instru-
ment is mounted and where his already fa-
nous astronomical work has been carried on.
This Monmouth telescope is reflector five
feet in diameter and is of The silver on glass
variety.
Mr.. Common formerly used a three-foot
reflector, which was constructed under his
Immediate supervision. 1t was an inStru-
ment OF remarkably fine figure and definition
With it he took his celebrated photograph of
the Orion nebula, which stands unrivaled in
excellence. and also many other celestial
views which have not as yet been surpassed
de recently disposed of his three-f0ot instru-
ment in order to make room for this larger
aud more powerful one, which bids fair to
far outstrip its predecessor.
The relative powers in the same classes of
instruments are easily calculated. for the
amount of light collected at the focus varies
in proportion to the square of the aperture.
thus, II we represent the light-gathering
power of the three foot mirror by nine, that
OF the fve-foot will be equal to twenty-five,
or In other words the latter is nearly three
times more powerful in this respect than the
former. This telescope is probably the most
powerful In light gathering capacity of any
ID.
which is six feet in aperture, alone is larger,
but that is composed of metallic compost
ton called spectrum metal, an alloy of tin
and copper, which Is much inferior in luster
to the silver surface of Mr.. Commons tele-
scope. This latter instrument also has the
advantage of being easily repolished or even
of having the silver film entirely removed
with nitric acid and recOated at a trifing
cost; whereas that of Lord Ross has to be
rehgured every time it is repolished, which
Is great disadvantage.
me construction OF mirror for large
reiiecting telescope is stupendous under
taking, and requires the utmost care and dex-
teflty, practice, and especially natural ap-
t s In order to bring the task to success
~ issue: Inc mode of operation is briefly
Ihls' flrst a disk of glass is procured of the
requisite diameter and thickness. The un-
sullled purity OF material and perfect homo
gently OF the mass which is absolutely nee.
essary in glass destined for the construction
OF reiractors. ana which adds greatly to their
cost, Is here unessential. Usually mirrors of
this kind are roughly concayed by grinding
with a convex tool composed of lead or cast
iron and using either sand or coarse emery
and water in the abrading process, until an
approximately true curve Is obtained. In
the case of this immense surface, however a
new expedienr was adopted which materially
lightened the laborious process. The glass
disk was fitted into lathe and turned to the
required curvature by means OF a diamond.
Fine emery and water is next used and a
smooth ground surface secured after which
1t is polished and figured by means of rough
water and pitch tool I his last process is
very delicate and the greatest care and
skill are necessary in order to arrive at the
best results.
These large mirrors, which are of compar
atiyely short focus and thus concentrate
great amount of light upon a relatively small
surface, are especially suited to nebua pho
Biography. The sensitive plate, in this work,
is usually exposed to the object to be photo.
graphed for several hours. The telescope
follows the object during the exposure by