The sun. [volume] (New York [N.Y.]) 1833-1916, 1897-08-15
DIR. PEATE'S GREAT MIRROR
THE LdRGE y SILTER- ON- GLASS
TELESCOPE REF ECTOR.


- Points or Superiority or Rrnectorn
OVor Rorrnctors - other Great spccuia.
From The Phiiaicyhid tr5Wc ledger.
II was on tho rather cquivocal date Of April
!, 1906, that the LcdRcr announced tho COM
mcnccmcnt at Grconvilic. Pa. by the Rcv. Dr.
John Pcato, OF the construction of n mammoth
spectrum, designed to serve MS tho light-gathor-
ing medium OF n rejecting telescope for tho
American University at Washington.
After two years OF arduous and absorbing
toll, involving the exercisc of skill Of the highest
grade, DIR. Peato advises the Lecturer Corrcspon-
dent In n private note that he has given tho
great mirror its last touches" l. e., in grind.
ing" and polishing and that 1t is ready for
silvcrinE, and for the farther and very impors
tant stage OF mounting"
Although rejecting telescopes are not un
known In the United States. the popular COM
prehcnsion Of tho term telescope Is mainly linz
tied to that class OF Instruments constructed
upon the spy-glass principle, viz, n tubc. having
Mt one end an object glass for gathering the
rays Of light coming from any object, and at tho
other nm eye piece or microscope for mngnifying
the image formed by the object glass. Such q
telescope is called n "rcfactor.' because
tho rays OF light falling upon the objective,
which consists of n double convex lens, are bcnt
to n focus by the Curving surface Of the glass.
In these days such u lens Is always compound.
usually consisting of two discs Though some
times of three. one of which Is OF crown glass,
double convex in shape, and the other OF flint
glass OF n pianoconcave form. This combine
ton is necessary to eliminate the troublesome
'chromatic alteration which cnsues when q
single lens is used. Inferior spy glasses and
opera glasses exhibit this feature in the color
fringcs which surround all objects.
1t was because of the existence of this color
alteration In the early days of rcfractors that
the rejecting telescope was devised. The trouble
is due to the fact that the rays of tho various
Colors which compOsc n beam of white light are
Of different degrees of rcfrangibility; and the
only method by which II could then be even
mcasurably overcome was to grind glasses hav-
ing very small apcratures of an immense focal
length, and dispensing altogether with n tube,
the object glass being mounted upon n tall pole
and controlled by n cord, while the eye piece was
attached to Il stand on the ground.
There is no such embarrassment in the refieo-
ton of light by q concave mirror. and Sir Isaac
Newton was among the first to introduce tele-
scopes upon this now principle. The specula. or
mirrors. were enormously heavy, being ground
of a metallic alloy, which was also exceedingly
brittle. (The spectrum Of the great Lord Rose
telescope weighs four tons.) And the labor Of
| properly shaping the surface, SO that all the rays
reflected would be focused at one point, was
prodigious, for In order to accomplish. this the
section of the mirror must be a parabola-
This, class of instruments was greatly i'm.
Sir William Herschel. who made a host Of
specula, OF constantly. increasing dimensions,
40 feet focus. with which he, discovered two
faint satellites Of Saturn: With one Of two feet
coyery of the dim planet Uranus.
The greatest Of the worlds telescopes created
i nPSn tine G'o Of fdyd K P%5issip as Pat 1ns ssIs91n irel
land, which is 6 feet in diameter, and occupies q
E Eassts er tgge ri that afe when lShn q bo Un he p had ,af fr %5. u
~ scampered through this vast cylinder when
While. however. refractors arc the real work
ing instruIncnts' Of astronomy. being more con-
vcniently handled and maintained in order.
there are some features of ASTRONOMICAL service
in which rcfectors are superior. Moreover, the
disparity in cost is very great. for the discs
four surfaces to be shaped. which must coincide
accurately II satisfactory results arc to be at
surface of the speculum need be free from faults.
while there is but one surface to grind.
In later years the metallic specula of reflect
ing telescopes have given placo to silver-on-
glass mirrors. which have produced very graft;
Tying results. When the speculum has received
lbs tina! figuring and polishing il is snbiected to
cave surface. and the thin film thus deposited 10
FURNISHED when thoroughly dry, being thereby
brilliancy.
In contrast with refractors rejecting tels-
scopes are considered to be equal in light-ghther-
ing power to objcctives of apcrtures say one
sixth less. but superior even to equal npertures.
perhaps. in dividing double stars, and certainly
SO in respect of achromatism. since the finest of
detectives is never absolutely achrornatic., But
qucnt furnishing and occasional renewal, and
the adjustments are too readily disturbed, whilS
the great mirror Is more subject TO Texture"
and consequent distortion OF the image, than q
compound lens. A flaw on the surface would also
be more troublesome than u similar defect In an
objective.
In "A shop attached to his residence at Green
vile. containing three rooms and a testing gall
lery. DIR. Peate has spent two years Of %continu.
Gus effort to properly iiEure a hnGe dise OF glass
| inches in thickness, The grinding tool had n
convcxity Of nine. siXtccnths OF an inch and POO
at the same distance from the centre. The power
employed was q gas engine, and emery OF Tari-
The great spectrum reposcd while being ground
upon u revolving table siNty-threS inches IN d.
which permitted The mirror to be raised on edge
for the purposc of tn" shadow test nthe test
or org wii ntdicvS; s a i"mp p' Was employGad,, around which
wns placed n metal shield or screen. with q
minute hole in il: The rays avs of Of light p nnS ed
cutting off OF which with n knife blade served
to determine the correctness Of the figure OF the
slightest change in the temperature will affect
thS mirror: but when 2 spGc1ium has success.
conditions OF atmosphere no fear, need be en-
tertaincd OF its performance on the heavenly
The patience involved in hundreds Of these
tests is inconeeivable. In the case Of the great
Ycrkcs lens. Mr.. Alvan e.. Clark assured the
writer that. in addition to the innuncrablc shop
tests. he had mounted that weighty and precious
than seventy timesl
DR.. Pcate has not yot attempted to silver his
prodigy. But unadorned in it's beautiful and
shining metallic dress. 1t has, nevertheless. an
incredible. light gathering power. The Doctor
writes: II can see a common pin and one of my
own gray hairs tied on it. more than ! 900 feet
from the mirror across the fields"
The reflection of the moon at the focal point
would be absolutely binding.
When the intricate and costly mounting is
ready for the spectrum IL will probably he used
in the Newfoundland style, l. G., It will rest upon
an adjustable bed at the bottom Of the, giant
tube, and the rays of light falling, upon ~ VIII
be reflected to tho sky end Of the tube. in tho
centre of which a plain mirror. placed at an
angle of forty.ive degrees. will elbow them out
at tho side OF the tube for magniilCation by an
eye piece. The observer. OF course. will 10 high
in tho air.
Tenderreet stumble on - Gold Mine.
From the Dcnitr HcpUhIicdn.