88 
THE YOUNG SCIENTIST. 
Common Telescopes.— IH* 
THE MODE OF OBSERVATION. 
AN ordinary telescope may be easily 
l^repared for use : to fix it on its stand ; 
to point it by means of the finder; to 
adjust the focus to the eye (remembering 
that different eyes require different ad- 
justments), are processes scarcely requir- 
ing instruction. But many mistakes may 
be made in detail ; and in this, as in every- 
thing else, there are various methods of 
doing the thing the wrong way. The 
present section will therefore consist of 
negative rather than positive directions, 
pointing out rather what should be 
4ivoided than what should be done. 
1. Do not begin by fixing the telescope 
in a warm room and opening the window. 
A boarded floor is bad, as every move- 
ment of the observer is liable to produce 
a tremor ; but the mixture of warm and 
cool currents at the window is worse ; it 
is an artificial production of the fluttering 
and wavering which, as naturally existing 
in the atmosphere, are such an annoyance 
to astronomers. If a window must be 
used, let it be opened as long beforehand 
as may be, and let the object-glass be 
pushed as far as possible outside; there 
should be no fire in the room ; and any 
■other windows, as well as the door, should 
be shut before .beginning %o observe : the 
nuisance may thus be sometimes abated ; 
but the right place is unquestionably out 
of doors. 
2. Do not wipe an object-glass or me- 
tallic speculum more than can possibly 
be helped. Hard as the materials are, 
scratching is a very easy process ; and the 
ultimate result of ordinary wiping may be 
seen in an old spectacle-glass held in the 
sunshine. The most valuable part of a 
good telescope deserves much more careful 
treatment ; and, if protected from dust and 
damp, it will very seldom require to be 
touched. Nothing but great carelessness 
would expose it to dust ; and the dewing 
of the surface may be almost always 
avoided. The object-glass or speculum, 
if kept in a cold place, should not be un- 
*From advance sheets of "Webb's Celestial 
Objects for Common Telescopes." 
covered, if possible, in a warmer air till 
it has gained something of its tempera- 
ture ; and it must be invariably closed up 
in the air in which it has been used before 
it is removed in-doors ; or, in either case, 
it may be dewed like a glass of cold water 
brought into a heated room. The object- 
glass, however, being much exposed to 
radiation, requires additional protection ; 
and this may be easily contrived. A tube 
of tin, pasteboard, or very thin wood, such 
as is used for hat boxes, or, best of all, 
calico stiffened with shellac and varnished, 
fitting on to the place whence the brass 
cap has been removed, and three or four 
times longer than wide, will, in general, 
keep the object-glass bright. The " dew- 
cap " must fit tight enough to stand firm, 
or it will bend down and intercept the 
light ; but not so tight as to cause trouble 
in removing it to put on the brass cap in 
the open air. It is better to blacken its 
interior — indeed, necessary, if of tin ; this 
may be done with lamp-black mixed with 
size or varnish, so as neither to show a 
gloss nor rub off; or a piece of black 
cloth or velvet may be glued or pasted 
inside it. A lining of blotting-paper is 
serviceable in heavy dew. A dew- cap on 
the finder will often save much trouble. 
Should it be necessary to leave the tele- 
scope for some time in the cold, a clean 
handkerchief thrown over the end of the 
dew-cap will be a complete safeguard. 
Should an object-glass or speculum be- 
come damp after all, do not close it up in 
that state; if the cloud of dew is very 
slight, it may quite disappear in a warm 
room, especially if exposed to a fire; if 
dense, however, it may leave a stain which 
ought to be quickly removed, as well as 
any little specks of dirt or dulness which 
will form, one knows not how. To do 
this, dust the dried surface first with a 
soft camel's hair pencil or varnishing 
brush, which will remove loose particles ; 
then use, very cautiously, a very soft and 
even piece of chamois leather, which has 
not been employed for any other purpose, 
and must be always kept in a wide- 
mouthed stoppered bottle or wrapped up 
from dust; or a very soft silk hand- 
kerchief (which Lassell uses for glass) 
preserved with similar care. But the 
