464 
POPULAE SCIENCE EEVIEW. 
of tlie large telescope. Thus, a star brought to the centre of 
the large field of the finder (indicated by the intersection of 
two fines placed at the focus of the eye-glass) is at, or very 
near, the centre of the small field of the large telescope. 
In searching with an alt-azimuth for an object not visible in 
the finder, the best plan is to narrow the field of research by 
selecting as pointers stars — whether conspicuous or not is 
indifferent, so long as they are visible in the finder, — as near 
as possible to the object sought. The “ straight-line pointing” 
of gnomonic maps and the “ true-angular pointing ” of 
stereographic maps are useful helps in finding; but if too 
large a part of the heavens is included in such maps, exces- 
sive scale- variation and distortion diminish their value. It is 
easy, in selecting maps, to determine the extent of these 
defects : — Examine the spaces included between successive 
meridians and parallels, near the edges and angles of the 
maps; these should not be very different either in size or 
shape from the corresponding spaces near the centre of the 
map. 
The quality of the stand has a very important influence on 
the performance of a telescope. In fact, a moderately good 
telescope, mounted on a steady stand, working easily and 
conveniently, will not only enable the observer to pass his 
time much more pleasantly, but will absolutely exhibit more 
difficult objects, than a better instrument on a rickety, ill- 
arranged stand. A good observing-chair is also a matter of 
some importance, the least constraint or awkwardness of 
position detracting considerably from the power of distinct 
vision. 
The object or the part of an object to be observed should 
be brought as nearly as possible to the centre of the field 
of view. Where there is no apparatus for keeping the 
telescope pointed upon an object, the best plan is so to 
direct the telescope, by means of the finder, that the object 
shall be just out of the field, and be brought by the earth's 
motion across the centre of the field. Thus the vibrations 
which always follow the adjustment of the tube will have 
subsided before the object enters the field. The object should 
then be intently watched during the whole interval of its 
passage across the field. 
It will be found that even in the worst weather for observa- 
tion, there are instants of distinct vision, 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 
applied. The telescopist should not be deterred from obser- 
