The herald., 1897-05-09
Getting changes in the brightness Of the

tecting changes in the brightness Of the
stars
This charting II accomplished by
means of Photographic telescope
which Is essentially a camera turned
by clockwork at such a rate as just neu-
tralizes the effect Of the earth's rota-
tion on its axis. The telescope is thus
pointed at any particular portion Of the
sky for the time necessary to obtain q
good photograph. The time required will
vary according to the brightness of the
object or region photographed. During
moonlight nights the plates are ex-
posed for approximately ten minutes, as
Ia longer exposure would 20 blacken
them az to render them useless for the
purpose intended. The plates chiefly
used are Sx10 inches in size, and each
photographs portion Of the sky equal
records the position, WHICH Is rApi-ss~u
in celestial latitude and longitude, and
also the times of starting and stopping
the exposure. He has.further to keep
a supply Of filled plate holders at hand
land to keep in good adjustment the
|means of which the camera or telescope
driving mechanism or clockwork, by
is kept constantly pointing accurately
at the came portion Of the sky during
the exposure Of the plate All this re-
quires the observer to be almost con-
stantly at the instrument. It has been
thought very desirable. therefore. to dc
"a large part of this Photographic chart
ling by mechanical means, and 1t. Is for
this purpose the instrument has been de-
signed at this observatory, and Is now
nearly completed.
las a result of the earth's motion In
space combined with its revolution on Its
axis. all portions Of the sky visible in
our latitude come at some time during
the year to the meridian. If we confine
our charting to zones lying on the merid.
ian. taking the regions as they drift
through this zone, we shall have GOV.
cred eventually all the regions which
are visible in our latitude The new
telescope is designed, therefore. to work
in this narrow belt or zone Of the eky
which has the meridian for its center
The telescope starts near the meridian
and follows a region for twenty minutes
at the end of which time 1t automatical
ly and quickly returns to its starting
point. During this return the telescope
is aleo automatically pointed ten de
grees further north or UP toward th'
zenith, thus pointing to region JUS
above the one last taken.
LAS 1t Is almost unavoidable that, ow
ing to cloudy weather and other causes
some regions get past the meridian with
out being observed, this telescope is pro
vided with an adjustment which will en-
able 1t to start, when required. one hou
or two hours west of the meridian an
thus recover regions which would other
wise be lost"
The use Of this instrument Is likely t
increase the usefulness Of Harvard oh
servatory to the astronomical world. al
though that institution has long hel
foremost place. It Is likely to mean ais
closer acquaintance with the heaven
than 1t has ever before been possible
attain, as this automatic photograph
Is certain to obtain q very different clas
of stellar pictures than the mere skill
man has heretofore secured.
It is expected that the new telescor
and the accompanying apparatus wi
have been placed in position and be
full working order by the first or mie
', die of June. A series of experiments wi
. then be attempted in the way of stella
photography which. 1t is believed, wi
, be attended with results Of exceeding IL
, terest and Importance.