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 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 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.