The Indianapolis journal. [volume] (Indianapolis [Ind.]) 1867-1904, 1883-10-02
MY TELESCOPE.
A LOng-Felt Want-How 1t Was Obtained
and BuiIt-Size, Cost and Magni-
flying Powers.

or $500 to buy it with was well ont of my
reach. At length, however, opportunities
came by which 1 could obtain 1920 to invest
in such property. I then began to devise
plans TO make the most of ito get all the
telescope 1 could for the money. knew
that good obiect-glass was the main thing.
Then how to get such a one, of the largest
size l could reach, was studied. l had heard
of Alvan Clark, near Boston, as an excellent
workman. The great telescope man of the
Washington Observatory said / had better
employ Clark. After what appeared sufi-
Gent correspondence, early in 1867 1 sent A.
Clark & Sons an order for four and one-half
inch obiectice of the best quality. 1t was to
be ready within five or six weeks, and would
cost 1885, as dollars were then counted-$135
gold, as now counted. Eye-pieces would cost
S. each. 1 ordered three of them. Some diff
ficulty in otaining the last payment was over
come, and about the most gratifying occa-
sion of my life was the arrival and sight (on
March 12) of glasses for siX-foot achromatic
telescope. l had already procured tube,
made of zinc, in the larger end of which [
placed the new treasure and, although it
was showing l soon had a pleasing view of
terrestrial objects mile or more distant. [
was also gratified to find the aperture of the
glass as inches instead of AS as or
dered adding about Sc to its value. They
"throwed in" S. eye-piece and one or two
other small articles which they knew would
need. | made an eye-piece of low power and
large field, and another one for a micrometer
Ito measure sun-spots, etc. six eye-pieces in
all-giving respective powers of about %, so,
100 $50 (this has the micrometer), 200 and
400 The tube is made tapering, 81 feet long,
81 inches diameter at the smaller end, and
5R at the larger end. 1t is painted bright
pale red on the outside and black inside
The outer portions of the mass of light en-
tering and passing through the object-glass
are bent and brought to focus (about q
inch diameter nearly seventy-one
inches from the objective. series
of rings were placed in the tube
about one foot apart and nearly the size of
the respective parts of the descending cone
of light to prevent scattering of the rays of
light and secure a more perfect image in the
focus This image is magnified by the eye
piece the same as an insect or other object is
magnified by a microscope. The more con-
vex and smaller the lenses are, the more
power we get. An eye-piece has two lenses
the one next the eye being smaller than the
one next the object. The latter is called the
field lens; if it is larger than the focal point
or image, the full size of the object (the
moon or whatever may be in the field of
view can ali De seen dG OIlCe. MY ~ eye
piece shows all of the full moon at one view,
but my 100 eye-piece does not, its field lens
being smaller than the focal image formed
by the object glass. There are two or three
kinds of mounting machinery to support the
tube and allow it to move freely in any di-
rection. After some study 1 decided that
equatorial mounting would be quite pre-
ferable to any other. 1t consists of
an axis, or axle, placed parallel with
the earths axis, i. e., in a north
and south position, with the north end ele-
waited 400, or equal to the latitude. This is
called the polar axis. Another one placed
across the north end and firmly attached to
it is called the declination axis The tele-
scope is placed across one end of this, and Il
balance weight at the other end. These axes
are of iron, nine inches long round and
smooth, and slightly tapering. Each end
(near the end) of both is loosely held in
thimbe so they turn, and admit the tele
scope OF pointing in any direction, and when
fixed on star will follow it in its daily mo-
ton simply by turning the polar axis A
graduated circle attached to the lower or
south end of the polar axis enables me to
place the telescope in any degree of right
ascension, and by one on the declination axis
I can point to any degree of defamation
north or south of the equinoctial. This
mounting is placed on heavy stand, made
of strong oak timber. Much study and work
were done before all this was completed.
And considerable shoemaking must be done,
too. So it was two weeks after the glasses
came before | was gratiied with splendid
views of the sun and stars. But it was pleas
ant TO obtain good views of test-objects, in
tended for larger telescopes-evidence that
my new glass was a very good one
| soon experienced difficulty in putting
the telescope in the exact range with star
so it would be in the field of view; although
I could see it with the naked eye. This
showed the necessity of a finder" so 1 put
the objective of an old spy-glass IN one end
of small tube (having already taken the
spy-glass tubes for the eye end of the tele-
scope) and made an eye-piece with cross-hairs
in tine center, to the other end, then attached
this little telesco9e to the smalleren of large
one. and parallel with it. 1t is very easy to
get star in field OF view OF the finder, then
bring it to the center-cross. when it is in the
field OF the telescope. This arrangement is
of exceeding convenience. An unsteady
condition of the air prevails so nearly all the
time that very high powers can seldom be
used with advantage. The only way to
know is TO be on the lookout and see when
the air is very good, then use it with high
powers. In general power of one hundred
is the best. If the air is very tremulous
fifty is plenty. By the one hundred power
l mean that the object is magnihed one
| hundred times across, and both ways, which
is actually 10, 900 times in area, the square of
one hundred. Now this makes very great
change in the appearance of an object, and
yet we are not apt to realize the full extent
| of it. There is a certain want of apprecla-
| ton, so to speak, it might be called an op-
teal illusion, by which an object
say planet-when magnifed just enough to
make it the size of the moon seems consid-
erably smaller. The apparent diameter of
Jupiter is about 40 seconds and that of the
moon 1, 300 seconds A power of fifty on Ju-
piter would make it 1000 seconds, some
larger than the moon. But the disk appears
less than that of the moon. However, if we
choose a time when the moon and planet are
near together, and look at Jupiter through
the telescope and the moon with the other
eye outside the telescope both at the same
time. we will see then that Jupiter looks as
large as the moon. In the first effort it is