Knoxville journal. [volume] (Knoxville, Iowa) 1874-1986, 1874-06-18

/ Chatham Island station, at the extreme
east, will be occupied by Mr.. E. Smith, as
chief astronomer, assisted by Mr.. Scott Mr..
Seebohme and Mr.. L. III. Ayne are the pho
| tographers.
Each of these parties, eight in all, will
| have three small houses, about twelve by
| twelve. They are all painted dull lead
color, and are so constructed that they
can be taken to pieces and packed on
board ship like ordinary lumber. On
arriving at their different destinations they
will be put up and the instruments placed
in them.
Each party is provided with an instru-
mental outfit, which has been prepared by
Prof. Hardness. The outfit includes an
equatorial, transit instrument, so con-
structed that it can be used as zenith
telegraph, clock with chronograph or
time-writer, two box chronometers, full
set of engineers instruments, magnet
ometer, complete Photographic outfit,
chest of carpenters tools, and appropriate
supplies of food and clothing for the
widely-different sections of the world
they are to occupy.
For long time many of the parties
who are to accompany the expedition
have been in training in the Naval OBJ
servatory, in the use of their various in
struments and in such processes as they
will go through with at their respective
stations instruction in photography has
been given by Prof. Henry Draper, who
has received prize medals for excellence
in the art from the Academy of Arts in
Paris.
Photography is applied because it holds
the most prominent place among the
methods of determining the angular dis
lance of the center of Venus from that of
the SUM- The objects sought to be accom-
plished are these:
FSrst-To form an image of the sun
with Venus on its disc, such an image
that from it the outlines of the images
that is to say, the points on the photo
graphic plates which correspond to the
centers of the two discs-can be fixed
with a superior degree of precision.
Second-The linear distance between
these points being determined in MILNE
tiers or other units Of length by means OF
the micrometer, the mean result OF deduc-
ing the angular distance to which this
linear distance corresponds, or knowledge
must be gained of the value of one MILL
meter in seconds of arc on each part of the
Photographic plate, and in each direction.
ThSrd-There must be on the plate
fixed line of reference, and from this line
deductions can be made of the angle of
position Of the two centers relating to the
circle of right ascension passing through
the two centers. This goes far enough for
the general reader. Questions of accu-
racy required in the measures of the size
of the image on the plate, of the modes OF
forming the solar image to be photo
graphed, of the heliosat, of the objective
and the tube, of the arrangements at the
focal points, of the exposing plate, of the
determination of the position of the planet
on the suns disc, and of the angle of po-
sitlon are left where they belong, with the
men advanced in the abstruse and intri-
cate analyses involved in them. All the
profoundest astronomer can do in the
premises is simply to accurately mark
the path of the planet across the sun, and
to note the real and exact time of its
various positions Any one on that day,
if at proper point, by looking through
piece of smoked glass will be able to see
what appears to the eye as mere spot
about three-eighths of an inch in diane
ter on the disc of the sun, which will
seem to be about twelve inches in diam
eter, and this spot will continue in sight
about four hours.