The San Francisco call., 1897-02-14

from its great size will be constructed and
mounted in sections. When the parts are
assem bled in place to form the complete
mirror it Will measure 200 feet in dame
ter. To mount so gigantic an affair
would require frame work of enormous
dimensions, and besides, any improve
ment of the whole would be extremely
difficult; certainly no delicacy of move
ment could be attained. The huge con-
cave mirror is therefore held in place by
framework of steel and brass, so arranged
as to compensate for expansion and con-
traction, and thus preserve the symmetry
of the mirror under changes of tempera-
ture. This framework rests upon tank,
or float which is supported in pond or
basin of water. By this means stability
OF equipoise is secured, the vibration of
the earth is reduced TO a minimum, and
by a we'll planned system of movable
weights, the mirror may be made to in
cline al desired angle, or turn from one
point of the heavens to another.
The image reflected from this giant
speculum mirror is received at the proper
focus by the lenses of the eyepiece, which
is situated 5000 feet distant from the sur
face of the mirror. This lengthy focal
distance Is necessary for the purpose of
keeping the reflected rays as nearly par
allel as possible, so that distortion at the
focal point my be diminished to a mini
mum and a finer definition obtained than
would be possible should the rays converge
at the acute angles.
Notwithstanding that our moon is called
dead" star and is supposed to have no
heat OF Its own, yet it reflects to US the
heat as well as the light of the sun, and
every stellar body sends some heat to the
earth. The concentrated rays OF the huge
mirror used in the new telescope would
convey sufficient neat from the moons
surface to destroy the eyesIght of any OBJ
server who had the temerity to look into
the unprotected eyepiece. To remedy this
translucent prism of rock salt is placed
in front OF the eyepiece between it and the
big mirror, which absorbs the heat rays.
1t. however, very materially dims the i'm
age OF the object under observation, and
when the telescope is used for photo
graphic purposes the salt prism is re-
moved.
To satisfactorily observe a celestial OBJ
iect, as a star, the telescope must be made
TO follow the STAR" that is, it must be
made to move in a contrary direction to
that OF the earth. In ordinary astronom-
lcai telescopes this is accom- plished by q
clock, whose works connect with the tele-
scope and keep it constantly pointed at
the object observed. But the giant re-
Hector is too large for this, and its gaze Is
therefore confined to o.objects in the zenith
or close TO it. Fortunately, the rotation
OF the earth upon her axis brings most of
the desirable heavenly objects to the
zenith at one time or the other. The huge
telescope here described is movable only
to very slight degree, and used as an OBJ
servation instrument for the human eve
would prove unsatisfactory. But the pho
tographic eye is the one intended for its
gaze. Photographic science has narrowed
down the length of time of exposure nee
essary to produce a perfect negative to the
one-ten-thousandth part of a second !;
whlle the perfection attained in the pro-
ducilon OF Photographic films make. IL a
comparatively simple task to obtain a.pic-
ture so ininitesimally small as to require
'he highest power microscope to make
visible to the human eye the photo
graohed view, which is, however, CON-
plete and clear in every detail.
USED as intended, the giant reflector
Will nave no eyepiece, but in its place
Will be specially constructed photo.
graphic camera, fitted with a system of