een with a smaller
instrument necessarily having less 'light grasp.' Yet this very excess
of light, which is the great advantage of a large instrument, is one of
the things that spoils the definition of faint planetary details; it
drowns them all out, or 'breaks them up.'

"Again, these large instruments are much more liable than smaller ones
to what is termed 'chromatic' and 'spherical' aberration; and this also
is detrimental to definition. No very large refractor is entirely free
from these defects.

"Another objection is that, in using such large and long-focussed
instruments, a much higher power must necessarily be employed than in
the case of smaller instruments. This high power magnifies all the
little movements and disturbances in our atmosphere to exactly the same
extent as it magnifies the object looked at, with the result that these
disturbances blur out all fine detail. The canal lines on Mars could
never be seen in such circumstances. If the object were looked at
through a smaller instrument, with lower power, it might be fairly well
seen, for the atmospheric disturbances would not be magnified to such
an extent as to spoil definition.

"There are very few nights in the year when these immense instruments
can be used to advantage on the planets, whilst a smaller instrument
might define well three or four nights out of every six. It is on record
that the user of Lord Rosse's great reflector stated that there were
only about three nights in the year when its best definition could be
obtained; and its use has produced very meagre results, compared with
what had been anticipated.

"It is also almost universally recognised that in using these great
instruments, whether for photography or for the visual observation of
fine detail, it is absolutely necessary to stop down the aperture to a
very large extent, by reducing it to about 12 inches in diameter or even
less. The big telescope is thus really converted into a small one of
long focus.

"There is, in addition, the acknowledged fact that nearly every
discovery of new detail on planets has been made with a comparatively
small telescope, although the same objects may have been under constant
observation for years with big telescopes. The new detail was never
noticed until after it had been seen with a smaller instrument, and
perhaps only then when atmospheric conditions were unusually good.

"As an instance, I may mention that the faint 'crape ring' of Sat