-telescope, one crater is immediately seen to be conspicuous beyond
all others, by reason of the brilliant rays or streaks that radiate from
it. This is the majestic _Tycho_, 17,000 feet in depth and 50 miles in
diameter (Plate X.). A peak 6,000 feet in height rises in the centre of
its floor, while a series of terraces diversity its interior slopes; but
it is the mysterious bright rays that chiefly surprise us. When the sun
rises on Tycho, these streaks are utterly invisible; indeed, the whole
object is then so obscure that it requires a practised eye to recognise
Tycho amidst its mountainous surroundings. But as soon as the sun has
attained a height of about 30 deg. above its horizon, the rays emerge from
their obscurity and gradually increase in brightness until the moon
becomes full, when they are the most conspicuous objects on her surface.
They vary in length, from a few hundred miles to two or, in one
instance, nearly three thousand miles. They extend indifferently across
vast plains, into the deepest craters, or over the loftiest elevations.
We know of nothing on our earth to which they can be compared. As these
rays are only seen about the time of full moon, their visibility
obviously depends on the light falling more or less closely in the line
of sight, quite regardless of the inclination of the surfaces, mountains
or valleys, on which they appear. Each small portion of the surface of
the streak must therefore be of a form which is symmetrical to the
spectator from whatever point it is seen. The sphere alone appears to
fulfil this condition, and Professor Copeland therefore suggests that
the material constituting the surface of the streak must be made up of a
large number of more or less completely spherical globules. The streaks
must represent parts of the lunar surface either pitted with minute
cavities of spherical figure, or strewn over with minute transparent
spheres.[8]

Near the centre of the moon's disc is a fine range of ring plains fully
open to our view under all illuminations. Of these, two may be
mentioned--_Alphonsus_ (19), the floor of which is strangely
characterised by two bright and several dark markings which cannot be
explained by irregularities in the surface.--_Ptolemy_ (20). Besides
several small enclosed craters, its floor is crossed by numerous low
ridges, visible when the sun is rising or setting.

21, 22, 23.--When the moon is five or six days old this beautiful group
of three crat