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Page 15
ASTRONOMICAL INSTRUMENTS.
"The eyes of the Lord are in every place."--_Proverbs_ xv. 3.
[Page 42]
"Man, having one kind of an eye given him by his Maker, proceeds
to construct two other kinds. He makes one that magnifies invisible
objects thousands of times, so that a dull razor-edge appears as
thick as three fingers, until the amazing beauty of color and form
in infinitesimal objects is entrancingly apparent, and he knows that
God's care of least things is infinite. Then he makes the other kind
four or six feet in diameter, and penetrates the immensities of space
thousands of times beyond where his natural eye can pierce, until he
sees that God's immensities of worlds are infinite also."--BISHOP
FOSTER.
[Page 43]
III.
_THE TELESCOPE._
Frequent allusion has been made in the previous chapter to discovered
results. It is necessary to understand more clearly the process by
which such results have been obtained. Some astronomical instruments
are of the simplest character, some most delicate and complex.
When a man smokes a piece of glass, in order to see an eclipse
of the sun, he makes a simple instrument. Ferguson, lying on his
back and slipping beads on a string at a certain distance above
his eye, measured the relative distances of the stars. The use
of more complex instruments commenced when Galileo applied the
telescope to the heavens. He cannot be said to have invented the
telescope, but he certainly constructed his own without a pattern,
and used it to good purpose. It consists of a lens, O B (Fig. 13),
which acts as a multiple prism to bend all the rays to one point
at R. Place the eye there, and it receives as much light as if it
were as large as the lens O B. The rays, however, are convergent,
and the point difficult to [Page 44] find. Hence there is placed at
R a concave lens, passing through which the rays emerge in parallel
lines, and are received by the eye. Opera-glasses are made upon
precisely this principle to-day, because they can be made
conveniently short.
[Illustration: Fig. 13.--Refracting Telescope.]
If, instead of a concave lens at R, converting the converging rays
into parallel ones, we place a convex or magnifying lens, the minute
image is enlarged as much as an object seems diminished when the
telescope is reversed. This is the grand principle of the refracting
telescope. Difficulties innumerable arise as we attempt to enlarge
the instruments. These have been overcome, one after another, until
it is now felt that the best modern telescope, with an object lens
of twenty-six inches, has fully reached the limit of optical power.
_The Reflecting Telescope_.
This is the only kind of instrument differing radically from the
refracting one already described. It receives the light in a concave
mirror, M (Fig. 14), which reflects it to the focus F, producing the
same result as the lens of the refracting telescope. Here a mirror
may be placed obliquely, reflecting the image at right angles to the
eye, outside the tube, in which case it is called the Newtonian
telescope; or a mirror at R may be placed perpendicularly, and send
the rays through [Page 45] an opening in the mirror at M. This form
is called the Gregorian telescope. Or the mirror M may be slightly
inclined to the coming rays, so as to bring the point F entirely
outside the tube, in which case it is called the Herschelian
telescope. In either case the image may be magnified, as in the
refracting telescope.
[Illustration: Fig. 14.--Reflecting Telescope.]
Reflecting telescopes are made of all sizes, up to the Cyclopean
eye of the one constructed by Lord Rosse, which is six feet in
diameter. The form of instrument to be preferred depends on the
use to which it is to be put. The loss of light in passing through
glass lenses is about two-tenths. The loss by reflection is often
one-half. In view of this peculiarity and many others, it is held
that a twenty-six-inch refractor is fully equal to any six-foot
reflector.
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