100 
THE YOUNG SCIENTIST. 
light, and usually smallest in the best 
telescopes. A very high power has, how- 
ever, so many disadvantages, in the difQ- 
culty of finding and keeping the object, 
the contraction of the field, the rapid 
motion of the image (in reality, the mag- 
nified motion of the earth), and the exag- 
geration of every defect in the telescope, 
the stand, and the atmosphere, that the 
student will soon learn to reserve it for 
special objects and the finest weather, 
when it will sometimes tell admirably. 
A very low power is apt to surround 
bright objects with irradiation, or glare. 
Experience in all these matters is the 
surest guide. 
It may be very useful to know the diame- 
ter of the field of each of our eye-pieces. 
This maybe obtained from the time which 
an object in or very near the equator takes 
in passing centrally through it; any star 
having but little declination will answer 
(7 Virginis and <^ Or ionis may be especially 
mentioned) or the moon or a planet in a 
corresponding i)osition. Several trials 
may be made, and the mean result in 
minutes and seconds of time multiplied 
by 15 will give the diameter of the field in 
minutes and seconds of arc, or space, at 
the equator. 
7. Do not be dissatisfied with first im- 
pressions. When people have been told 
that a telescope magnifies 200 or 300 times, 
they are often disappointed at not seeing 
the object apparently larger. In viewing 
Jupiter in opposition with a power of only 
100, they will not believe that he appears 
betw^een two and three times as large as 
the moon to the naked eye ; yet such is 
demonstrably the case. There may be 
various causes for this illusion — want of 
practice— of skij-room, so to speak— of a 
standard of comparison. A similar disap- 
pointment is frequently felt in the first 
impression of very large buildings; St. 
Peter's at Eome is a well-known instance. 
If an obstinate doubt remains, it may be 
dissipated forever when a large planet is 
near enough to the moon to admit of both 
being viewed at once - the planet through 
the telescope, the moon with the naked 
eye. 
8. Do not lose time in looking for ob- 
jects under unfavorable circumstances. A 
1 very brilliant night is often worthless for 
I planets or double stars, from its blurred 
jor tremulous definition; it will serve, 
however, for grand general views of bright 
groups or rich fields, or for irresolvable 
nebulae, which have no outlines to be de- 
ranged ; a hazy or foggy night will blot 
out nebulae and minute stars, but some- 
times defines bright objects admirably; 
never condemn such a night untried. 
Twilight and moonlight* are often advan- 
tageous, from the diminution of irradia- 
tion. Look for nothing near the horizon, 
unless, indeed, it never rises much above 
it ; nor over, or to the leeward of a chimney 
in use, unless you wish to study the effect 
of a current of heated air. If you catch a 
really favorable night, with sharp and 
steady vision, make the most of it ; you 
will not find too many of them. Smyth, 
who thinks our climate has been unfairly 
depreciated, says : " Where a person will 
look out for opportunities in the mornings 
as well as evenings, and especially be- 
tween midnight and daybreak, he will 
find that nearly half the nights in the year 
may be observed in, and of these sixty or 
seventy may be expected to be splendid." 
But ordinary students must of course take 
their chance, with their fewer opportuni- 
ties. With due precaution as to dress, 
nothing need be feared from " night air;" 
that prejudice is fully confuted by the 
well-known longevity of astronomers, 
even of such as have habitually protracted 
their watchings 
Till the dappled dawn doth rise. 
9. In examining faint objects, do not 
prepare the eye for seeing nothing, by 
dazzling it immediately beforehand with 
a lamp, or white paper. Give it a little 
previous rest in the dark, if you wish it to 
do its best.t 
10. When a very minute star or faint 
nebula is not to be seen at once, do not 
give it up without trying oblique or averted 
vision, turning the eye towards the edge 
*Secchi has found the detail of the Great Nebula 
in Orion much more visible in moonlight, which 
is also known not to obliterate even such objects 
as the satellites of Mars and Uranus, or some of 
; the minuter comites of double stars. 
tHerschel II , when about to verify his father's 
observations on the satellites of Uranus, prepared 
his eye with excellent effect, by keeping it in 
utter darkness for a quarter of an hour. 
