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POPULAR SCIENCE REVIEW. 
dimensions. Nevertheless, much can he done with an instru- 
ment of even moderate size. Messier discovered some score of 
comets, and upwards of a hundred nebulae and clusters, with 
a five-feet telescope. The great map of the moon — the 
standard one by Baer and Maedler — was delineated by an 
instrument of scarcely greater power. M. Goldschmidt has 
detected upwards of a dozen of planets with instruments 
ranging from a common opera-glass to telescopes of about 
four inches in aperture ; but in this case a most extraordinary 
keenness of sight must aid the unwearied diligence and 
patience of the observer. Although Maedler states that only 
two hundred nebulae can be detected with a telescope of five 
inches aperture, yet D’ Arrest assures us that he can see that 
number with a common hand-telescope or comet-seeker, whilst 
with an instrument of four inches aperture upwards of a 
thousand are visible. A great deal, in such cases, depends 
upon practice ; but a few precautions being taken, the amateur 
will, in a short time, be encouraged to proceed, by looking at 
the same objects at different times. Probably, he may notice 
that some stars which were pointed out to him as double, 
appear only as hazy and ill-defined single ones on one evening, 
whilst a few nights afterwards, with the sky apparently no 
clearer, they may be transformed by some magic process into 
what he had been originally apprized of. The state of the 
atmosphere exerts a wonderful influence on the definition of 
objects. Herschel, we believe, only allowed a hundred hours 
of superfine observing weather during the year. Secchi (at 
Pome) confesses he is more fortunate, and owns that under the 
cerulean sky of Italy he is favoured with a clear fifteen nights 
throughout the year. And yet those two observers have been 
constantly on the watch for those happy moments, not one of 
which, the latter tells us, should be lost, but the telescope 
at once directed to the faintest and minutest objects. We have 
ourselves found, that the best definition and brightness exists 
when the wind is from the damp southerly and westerly 
quarters, and the worst when from the dry north and east, 
particularly the latter. It is important to avoid placing the 
telescope at a window, especially when the atmosphere within 
is of a higher temperature than without ; and Herschel goes 
so far as to say that no roof with an opening, or even 
shady places in the open air, should be allowed. The same 
authority tells us that sudden changes from frost to thaw, or 
vice versa, are bad for definition. Above all, good definition 
need not be expected near the horizon, where undulations, 
mists, and scintillations give a boiling motion at the best of 
times, and render observation impossible. It is necessary, 
above all things, to keep both the object-glasses and eye-pieces 
