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 deg. 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