highest powers do not necessarily give the best views of celestial
objects. High powers in all cases increase the difficulty of
observation, since they diminish the field of view and the illumination
of the object, increase the motion with which (owing to the earth's
motion) the image moves across the field, and magnify all defects due to
instability of the stand, imperfection of the object-glass, or
undulation of the atmosphere. A good object-glass of three inches
aperture will in very favourable weather bear a power of about 300, when
applied to the observation of close double or multiple stars, but for
all other observations much lower powers should be used. Nothing but
failure and annoyance can follow the attempt to employ the highest
powers on unsuitable objects or in unfavourable weather.

The greatest care should be taken in focussing the telescope. When high
powers are used this is a matter of some delicacy. It would be well if
the eye-pieces intended for a telescope were so constructed that when
the telescope is focussed for one, this might be replaced by any other
without necessitating any use of the focussing rack-work. This could be
readily effected by suitably placing the shoulder which limits the
insertion of the eye-piece.

It will be found that, even in the worst weather for observation, there
are instants of distinct vision (with moderate powers) during which the
careful observer may catch sight of important details; and, similarly,
in the best observing weather, there are moments of unusually distinct
vision well worth patient waiting for, since in such weather alone the
full powers of the telescope can be employed.

The telescopist should not be deterred from observation by the presence
of fog or haze, since with a hazy sky definition is often singularly