The Universalist. [volume] (Chicago [Ill.]) 1884-1897, 1897-09-11

science the possession of an instrument
which need fear no comparisons with
the best that Europe and America can
show. How valuable this new acquisi-
ton is likely to prove may be judged
from the fact that it is precisely twice
the size-both in aperture and in local
length-of the largest Photographic
telescope hitherto possessed by the oh.
servatory-viz.. the photographic equa-
torial, which is so successfully taking it's
share in the international Photographic
survey of the heavens now in progress.
The new telescope will form a worthy
complement to the big % inch refractor
completed few years ago, the arrange
ments of which are more particularly
adapted to visual observation.
Ii it be true that photography is the
most important addition that has been
made to the resources of the astrono-
mer since the invention of the telescope
itself, it is equally true that it's adoption
has appreciabIy added to the difficulties
and responsibilities of the telescope
maker. The reason for this is simple
enough. With a telescope used only for
observations with the eye, stability of
mounting and perfect steadinees in mo-
ton, however desirable are not so abso-
lutely necessary as they are when pho
Biography is employed. If, owing to
some imperfection in the mechanism
the telescope does not exactly follow
the motion of the star under observa-
ton, which in consequence movee off its
proper position in the field of view, the
observer can easily bring it back again
without any harm being done. the &C.
tion of the eye being. as it we're, sus-
pended during the time that the object
is off the wires Ct the micrometer. But
with the untiring eye of the photograph
ic plate continuously receiving impress
sions at the END of the tube it is diff
ferent matter.
The plate cannot, like the human OBJ
server. reject those impressions which
reach it when the ar is out of the sci.
ected position on the field, but must
impartially record every position of the
image so long as it is in the field at all.
Hence the result of defects in the
mounting or of irregularities in the
driving clock will be the production of
unsatisfactory photographs. with blurred
and distorted pictures.
In the case of the new telescope,
which has been erected in the new Ply
steal Observatory building, every pre-
caution has been taken to insure
mounting as rigid and free from tremor
as possible. Ia order to obtain the
steadiness of very broad base, the in
strument is placed on an arch joining
the tops of two solid pierre, to which the
lower part of the whole building is
firmly tied.
The massive equatorial mounting it.
self is on the model of that which has
given excellent results with the ID inch
photographic equatorial, though several
improv have been introduced by
Sir Howard Grub, the maker of both
instruments. OF these the most notable
is the modification of the frame, by
which complete circumpolar motion of
the instrument is rendered possible,
even when it is pointing to the pole. 1t
is usually the case that telescope is
prevented by the projection of the
frame from following star across the
merid ian. The reversal which is thus
necessitated when the star under ex-
animation passes from the east to the
west OF the meridian is obviously great
disadvantage for Photographic work,
since it may involve interrupting the ex-
posture of the plate. The Thompson
equatorial is free from this drawback,
and can follow any star throughout its
course in the heavens until it has set
below the horizon.
Motion is communicated to the polar
axis by means of very carefully cut
endless screw. working on toothed
SONoOAf AAOh q uuIbAcQ Univ
ing clock. which is provided with sem
sitive FRACTIONAL governor. and is cleo
trically controlled on Sir Howard
Grubb's we'll known plan, by an indie
pendent pendulum, through an ingeni-
Gus, and somewhat complicated system
of differential gearing The clock is
driven by weight of twelve hundred
weight. falling foot in every eight
minutes, and the observer is saved the
trouble of attending to it by small
electric motor, which is arranged to
keepit continuously wound up, without
in any way affecting the going of the
ock.-London Times.