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Page 16
The mounting of large telescopes demands the highest engineering
ability. The whole instrument, with its vast weight of a twenty-six-inch
glass lens, with its accompanying tube and appurtenances, must be
pointed as nicely as a rifle, and held as steadily as the axis
of the globe. To give it the required steadiness, the foundation
on which it is placed is sunk deep in the earth, far from rail or
other roads, and no part of the observatory is allowed to touch
this support. When a star is once found, the earth swiftly rotates
the telescope away from it, and it passes out of the field. To
avoid this, clock-work is so arranged that the great telescope
follows the star by the hour, if required. It will take a star at
its eastern rising, and hold it constantly in view while it climbs
to the meridian and sinks in the west (Fig. 15). The reflector
demands still more difficult engineering. That of Lord Rosse has
a metallic mirror [Page 46] weighing six tons, a tube forty feet
long, which, with its appurtenances, weighs seven tons more. It
moves between two walls only 10� east and west. The new Paris
reflector (Fig. 16) has a much wider range of movement.
[Illustration: Fig. 15.--Cambridge Equatorial.]
[Illustration: Fig. 16.--New Paris Reflector.]
_The Spectroscope._
A spectrum is a collection of the colors which are dispersed by
a prism from any given light. If it is sunlight, it is a solar
spectrum; if the source of light is a [Page 49] star, candle,
glowing metal, or gas, it is the spectrum of a star, candle, glowing
metal, or gas. An instrument to see these spectra is called a
spectroscope. Considering the infinite variety of light, and its
easy modification and absorption, we should expect an immense number
of spectra. A mere prism disperses the light so imperfectly that
different orders of vibrations, perceived as colors, are mingled. No
eye can tell where one commences or ends. Such a spectrum is said to
be impure. What we want is that each point in the spectrum should be
made of rays of the same number of vibrations. As we can let only a
small beam of light pass through the prism, in studying celestial
objects with a telescope and spectroscope we must, in every
instance, contract the aperture of the instrument until we get only
a small beam of light. In order to have the colors thoroughly
dispersed, the best instruments pass the beam of light through a
series of prisms called a battery, each one spreading farther the
colors which the previous ones had spread. In Fig. 17 the ray is
seen entering through the telescope A, which renders the rays
parallel, and passing [Page 50] through the prisms out to telescope
B, where the spectrum can be examined on the retina of the eye for a
screen. In order to still farther disperse the rays, some batteries
receive the ray from the last prism at O upon an oblique mirror,
send it up a little to another, which delivers it again to the prism
to make its journey back again through them all, and come out to be
examined just above where it entered the first prism.
[Illustration: Fig. 17.--Spectroscope, with Battery of Prisms.]
Attached to the examining telescope is a diamond-ruled scale of glass,
enabling us to fix the position of any line with great exactness.
[Illustration: Fig. 18.--Spectra of glowing Hydrogen and the Sun.]
In Fig. 18 is seen, in the lower part, a spectrum of the sun, with
about a score of its thousands of lines made evident. In the upper
part is seen the spectrum of bright lines given by glowing hydrogen
gas. These lines are given by no other known gas; they are its
autograph. It is readily observed that they precisely correspond
with certain dark lines in the solar spectrum. Hence we easily
know that a glowing gas gives the same bright lines that it absorbs
from the light of another source passing through it--that is, glowing
gas gives out the same rays of light that it absorbs when it is
not glowing.
The subject becomes clearer by a study of the chromolithic plate.
No. 1 represents the solar spectrum, with a few of its lines on an
accurately graduated scale. [Page 51] No.3 shows the bright line of
glowing sodium, and, corresponding to a dark line in the solar
spectrum, shows the presence of salt in that body. No. 2 shows that
potassium has some violet rays, but not all; and there being no dark
line to correspond in the solar spectrum, we infer its absence from
the sun. No.6 shows the numerous lines and bands of barium--several
red, orange, yellow, and four are very bright green ones. The lines
given by any volatilized substances are always in the same place on
the scale.
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