y-opened pupil, it may,
however, happen that the image is not bright enough to excite the
sensation of vision. Here the telescope comes to our aid: it catches all
the rays in a beam whose original dimensions were far too great to allow
of its admission through the pupil. The action of the lenses
concentrates those rays into a stream slender enough to pass through the
small opening. We thus have the brightness of the image on the retina
intensified. It is illuminated with nearly as much light as would be
collected from the same object through a pupil as large as the great
lenses of the telescope.

[Illustration: Fig. 1.--Principle of the Refracting Telescope.]

In astronomical observatories we employ telescopes of two entirely
different classes. The more familiar forms are those known as
_refractors_, in which the operation of condensing the rays of light is
conducted by refraction. The character of the refractor is shown in Fig.
1. The rays from the star fall upon the object-glass at the end of the
telescope, and on passing through they become refracted into a
converging beam, so that all intersect at the focus. Diverging from
thence, the rays encounter the eye-piece, which has the effect of
restoring them to parallelism. The large cylindrical beam which poured
down on the object-glass has been thus condensed into a small one, which
can enter the pupil. It should, however, be added that the composite
nature of light requires a more complex form of object-glass than the
simple lens here shown. In a refracting telescope we have to employ what
is known as the achromatic combination, consisting of one lens of flint
glass and one of crown glass, adjusted to suit each other with extreme
care.

[Illustration: Fig. 2.--The Dome of the South Equatorial at Dunsink
Observatory Co Dublin.]

[Illustration: Fig. 3.--Section of the Dome of Dunsink Observatory.]

The appearance of an astronomical observatory, designed to accommodate
an instrument of moderate dimensions, is shown in the adjoining figures.
The first (Fig. 2) represents the dome erected at Dunsink Observatory
for the equatorial telescope, the object-glass of which was presented to
the Board of Trinity College, Dublin, by the late Sir James South. The
main part of the building is a cylindrical wall, on the top of which
reposes a hemispherical roof. In this roof is a shutter, which can be
opened so as to allow the telescope in the interior to obtain a view of
the heav