The Helena independent. [volume] (Helena, Mont.) 1875-1943, 1891-11-20
'optick tub" the object at present held in
view is of q somewhat more practical nature,

''optick tube" the object at present held il
view is of a somewhat more practical nature.
n deals more with how to use it from the
piazza or housetop i'll the pursuit of a more
interesting knowledge of the heavenly
bodies than can be had with unaided vision,
One is frequently at a loss to know what
are the chief differences between the astro-
nomical and the terrestrial instruments,
and how to make the one telescope answer
| the dual purpose. While there are many
features in the grasses used by professionals
that are not found in those in general SER
vice, it ia fairly within reasonable bounds
to say that the primary dissimilarity is in
the eye-piece. It makes not a particle of
difference to the whether the
moon, sun. planet, or star is upside down
or right-side-left-side whereas tho ordinary
mortal prefers seeing terrestrial or
marine objects right side up. Every or
binary lens possesses the great disadvan-
tage of having both chromatic and spher-
ical alteration. which must be corrected
before satisfactory fine observing can be
attempted. This difficulty is ordinarily
overcome by using two lenses of such focal
each counteracts the defects of the other.
Rejecting telescopes operate better in the
open air, or. at least, in a situation where
they are nearly sheltered from the wind.
From their form they are always mounted
On lows stands, and can easily be covered
with a fiat roof not more than six feet
high. The kind of observatory most suited
to them is. therefore, one with u roof that
runs back over at least one-half of the
DOME. Refractors are necessarily MOUNTED
on much higher stands, so that they require
q drum-shaped roof. Either of these can
be erected at moderate cost on the house
top or on firm ground.
The most suitable series of magnifying
powers for celestial telescopes are fifteen,
thirty, forty-ive, sixty, and ninty to the
inch of aperture. A very ~low-power eye
piece, with a large field of view and mag
nifylng about forty diameters, is useful for
finding objects. The greater number of
stars require the power of fifteen to the
inoh Of aperture; or, if bright enough to
boar it. the power of forty to tho inch may
be used. For the sun the power of thirty
to the inch is the only one that will be Of
muchs ervice. Mercury and Venus at day
will satdrn; but the satellites of Saturn
will be best seen with the next higher
power. Jupiter, Uranus, and Neptune
should have forty-ave to the inch, and at
times Jupiter should have tho next higher
power. Mars generally requires a power OF
sixty to the inch to show the markings we'll.
and this must also be used with Mercury and
Venus after sunset. For the moor a
power of forty-five will be found best for
objects unless seen under high illumination,
as whom the moon approaches full, in which
case sixty or even ninety will be required.
Occasionally. in very steady air, a telescope
will bear still higher power than these, but
even then but very little more can be made
out.
In using ones telescope the first thing to
be remembered is that if anything be worth
watching it is worth noting. Every obser-
walton should be accompanied by full de-
tails az to time, weather, etc. Whenever
possible q sketch should accompany the de-
scription. No matter how rough or imper-
feet this sketch may be, as long as it is &C.
curate the artistic features may be done
away with. Dimensions and coloring are
always of Importance. and the data is a
great necessity. for without it the observa-
ton is almost valueless.
Beware of cheap Johns" If one wishes
\ really good instrument he must be pre-
pared to pay 4 reasonable price for it.