ong branching
defiles.

BRIGHT RAY-SYSTEMS.--Reference has already been made to the faint light
streaks and markings often found on the floors of the ring-mountains and
in other situations, and to the brilliant _nimbi_ surrounding some of the
smaller craters; but, in addition to these, many objects on the moon's
visible surface are associated with a much more remarkable and
conspicuous phenomenon--the bright rays which, under a high sun, are seen
either to radiate from them as apparent centres to great distances, or,
in the form of irregular light areas, to environ them, and to throw out
wide-spreading lucid beams, extending occasionally many hundreds of miles
from their origin. The more striking of these systems were recognised and
drawn at a very early stage of telescopic observation, as may be seen if
we consult the quaint old charts of Hevel, Riccioli, Fontana, and other
observers of the seventeenth century, where they are always prominently,
though very inaccurately, portrayed. The principal ray-systems are those
of Tycho, Copernicus, Kepler, Anaxagoras, Aristarchus, Olbers, Byrgius A,
and Zuchius; while Autolycus, Aristillus, Proclus, Timocharis, Furnerius
A, and Menelaus are grouped as constituting minor systems. Many
additional centres exist, a list of which will be found in the appendix.

The rays emanating from Tycho surpass in extent and interest any of the
others. Hundreds of distinct light streaks originate round the grey
margin of this magnificent object, some of them extending over a greater
part of the moon's visible superficies, and "radiating," in the words of
Professor Phillips, "like false meridians, or like meridians true to an
earlier pole of rotation." No systematic attempt has yet been made to map
them accurately as a whole on a large scale, for their extreme intricacy
and delicacy would render the task a very difficult one, and, moreover,
would demand a long course of observation with a powerful telescope in an
ideal situation; but Professor W.H. Pickering, observing under these
conditions at Arequipa, has recently devoted considerable attention both
to the Tycho and other rays, with especially suggestive and important
results, which may be briefly summarised as follows:--

(1.) That the Tycho streaks do not radiate from the apparent centre of
this formation, but point towards a multitude of minute craterlets on its
south-eastern or northern rims. Similar craterlets occur on the rims of
other