The times. [volume] (Washington [D.C.]) 1897-1901, 1901-01-13

continued.
a system of electric control of the going
of the driving clock of the telescope by
means OF a sidereaI clock was introduced,
the simplest one being designed by Rus
sell of Sydney, a most elaborate one by
Grubb of Dublin.
Another application of the method of
long exposures has been the discovery or
minor planets by the trails impressed by
their motion among the stars on the pho
tographic plates on which the images of
both are impressed.
complete spectroscopic survey of the
stars by means of photography was COM
menced in 1886 at Harvard College, as a
memorial to Draper who died while he
was laboring diligently and successfully in
securing advances in astro-physical endui-
ries. To carry on this work at Harvard
Prof Pickering wisely reverted to the
method first employed by Fraunhofer and
utilized by Respighi and another in 1871,
Of placing prisms in front of the object
glass,
In the photographing OF stellar spectra
by means of objective prisma- the driving
clock of the telescope must not go exactly
at sidereaI rate, but at certain speeds de-
pending on the brightness and position
Of the star under examination.
This is necessary because the image of
the spectrum of a star on the photograph
is only thin line in which it Is impossi-
BILE to see the spectral lines: the spectrum
must be broadened. and this is accom-
plished by making the star image trail
to a certain degree on the plate. This
clock. the rate of which fs-made to vary.
In this way the trail of a spectrum of a
star on the Photographic plate is always
obtained of the same width, while the
density of the image is made fairly con-
stant by increasing the rate for bright
stars and decreasing It for fainter ones.
In this way speetra of the brighter stars
rivaling in perfection and detail those OBJ
tanned of the spectrum of the SIN itself
thirty years ago have been obtained. Such
photographs have rendered a minute
chemical classification of the stars possl-
BILE.
One of the most interesting applications
of photography to spectrum analysis dur
ing the latter part of the century has been
the utilization by Messrs. DesIandres and
that by employing photography images of
the sun and It's surroundings can be OBJ
tanned in light on one wave length: in
this way We can study the distribution OF
any one of the chemical constituents of
the sun separa teIy, and note its behavior,
not only on the sun Itself, but in the at
mosphere which enfolds the disk.
1t is strange that In spite Of the sugges-
lions Of Faye. and others after him. one
of the great advantages of the employ
ment of photography in astronomical
work, namely, the abolition of personal
equation has so far been almost entirely
neglected. What personal equation Is
can be perhaps illustrated by considering
an observer who is observing tine transit
of star over the wires in a transit in
s;rument-
His object is to note the exact time. to
a fraction of a second. when a star passes
each wire; and this Is done by listening to
the beats of a clock near at hand and
estimating the fractions. Some observers
constantly note the time either a little in
advance or a little later than the actual
time, and this small difference between
the observer and the true times is more
or less constant for each observer. This
difference has to be taken into account
for every observation. Even the use of
the chroaograph in transit work, by which
the observation is electrically recorded,
| does not entirely eliminate the error The
Photographic method of transit work has
been experimental on, but, SO far as
| know, it has not yet been used at more
than one or two observatories. It will
doubtless eventually rid US of personal
equation entirely. for the star image may
be photographed and the time recorded by
the same current of electricity.
At the end of the century We may al
most say that except in relation to the
work of meridional observatories, photo
graphic methods of recording observations
are becoming exclusively used One of the
cases in which its utility is most in evil
| dence is in the matter of eclipse observa-
tons. Spectra of the suns surroundings
containing thousand lines are taken in
a second of time, thus replacing five or
six doubtful eye observations by a wealth
of results which have enabled the recent
vast progress IS be secured
CATALOGUES.
Catalogues of the stars were among
the first scientific records started by man.
| and so long as only the naked eye was
used the work was not difficult as only
approximate positions were attempted
even by Hipparchus; but long before the
eighteenth century dawned the problem
was entirely changed by the invention of
the telescope and by the provision of ac-
oil ri only II
better positions be recorded but the num-
Der of stars to be catalogued was enor-
mously increased, and, Furthermore. other
objects, nebulae. presented themselves in
considerable numbers.
In 1891 the star catalogues chiefly relied
on were those of LacailIe, containing
about 5000 stars, scattered over the whole
heavens.
Maskelyne, who was then Astronomer
Royal, had published in 1870 catalogue
of thirty-six fundamental stars chiefly for
the purposes of navigation. The first
great catalogue of the century was
the Fundamental Assronomiae of Bessel,
produced in 1918. This contained 3.2s2
stars The Bonn "Durchmusterung. &"
with its catalogue of 32i, 19S stars in the
northern hemisphere and the correspond
ing atlas published in 1s57-63 was the next
memorable achievement in this direction.
For it we have to thank Bessel and
Highlander and a perfect system OF
work.
Another monumental catalogue dealing
with the stars in the southern heavens
has been that of the southern stars OBJ
III