470 
POPULAR SCIENCE REVIEW. 
next January, for instance, a large portion of tlie northern 
hemisphere will be visible. When Mars looks ruddiest, the 
spots on his surface are most clearly visible — probably be- 
cause at such times the atmosphere of the planet is freer 
from mist or vapour. 
Jupiter and Saturn are, of course, most interesting objects 
to the astronomical observer, though the close scrutiny to which 
they have been subjected by our leading observers may deter 
the amateur from that persistent and systematic observation 
which is alone likely to be rewarded with valuable results. 
Yet Mr. Grover, with a telescope only two inches in aperture, 
has detected a phenomenon in Saturn which, I believe, had 
escaped the notice of more powerfully armed observers, — the 
presence/ namely, of a penumbra surrounding the shadow of 
the planet on the ring. For a long time Saturn will not be 
favourably situated for observation, owing to his low altitude 
when in opposition ; and nearly twenty years must elapse 
before the planet will again exhibit his ring-system (as in 1856) 
fully opened and well elevated above the horizon. 
Double and multiple stars supply at all times a fertile field 
of labour and research to the astronomical observer. They 
also afford a ready means of testing (1) the performance of 
his telescope, and (2) the state of the atmosphere on each 
night of observation. It is well to begin each such night 
with preliminary observations of a few well-known double 
stars, suitably placed as respects altitude, and to enter the 
results of such observations immediately above the notes of 
the evening^ work. The following are among the tests 
given by Admiral Smyth for the performance of a telescope ; 
but it must be remembered that they were obtained by 
diminishing the aperture of a 6-inch telescope having a focal 
length of 8* feet, so that as far as the smaller apertures 
are concerned a somewhat better defining power was probably 
obtained than similar apertures with the usual proportionate 
focal lengths would give. 
A 2 -inch aperture, with powers of from 60 to 100, should 
exhibit — 
a Piscium (3". 7). I 8 Cassiopeise (9".5), mag. (4 and 7-|)« 
y Leonis (2". 9). | Polaris (18''.6), mag. (2J and 9D- 
A 4-inch, powers 18 to 120 — 
£ UrsseMajoris (2".7). <r Cassiopeise (3".l), mag. (6 and 8). 
y Ceti (2". 6). 8 Geminorum (7".l), mag. (4 and 9). 
A 6-inch, powers 240 to 300, — 
X Ophiuchi (1"4). a Arietis (30". 0), mag. (3 and 11). 
20 Draconis (0".7). K Geminorum (6".l), mag. (4 and 10). 
