HOW TO WORK WITH THE TELESCOPE. 465 
vation by the presence of fog or haze, since with, a hazy sky 
definition is often singularly good. 
The observer must not expect distinct vision of objects 
near the horizon. Objects near the eastern horizon during 
the time of morning twilight are especially confused by at- 
mospheric undulations ; in fact, early morning is a very 
unfavourable time for the observation of all objects. 
A telescope should not be mounted within doors, if it can 
be conveniently erected on solid ground, as every movement 
in the house will cause the instrument to vibrate unplea- 
santly. Further, if the telescope be placed in a warm room, 
currents of cold air from without will render observed objects 
hazy and indistinct. In fact. Sir W. Herschel considered 
that a telescope should not even be erected near a house or 
elevation of any kind, round which currents of air are likely 
to be produced. If a telescope be used in a room, the tem- 
perature of the room should be made as nearly equal as 
possible to that of the outer air. When used out of doors a 
“ dew-cap,” that is, a tube of tin or pasteboard, some ten or 
twelve inches long, should be placed on the end of the in- 
strument, so as to project beyond the object-glass. This 
tube should be blackened within, especially if made of metal. 
After use, says old Kitchener, the telescope should be kept 
in a warm place long enough for any moisture on the object- 
glass to evaporate. If damp gets between the glasses it 
produces a fog (which opticians call a “ sweat”), or even a 
seaweed-like vegetation, by which a valuable glass may be 
completely ruined. 
The observer should not leave to the precious hours of the 
night the study of the bearing and position of the objects he 
proposes to examine. This should be done by day, an arrange- 
ment which has a two-fold advantage — the time available for 
observation is lengthened, and the eyes are spared sudden 
changes from darkness to light, and vice versa. Besides, the 
eye is ill-fitted to examine difficult objects, after searching 
by candle-light amongst the minute details recorded in maps 
or globes. Of the effect of rest to the eye we have an 
instance in Sir J. HerchePs rediscovery of the satellites of 
Uranus, which he effected after keeping the eye in darkness 
for a quarter of an hour. Kitchener, indeed, goes so far 
as to recommend (with a crede experto) an interval of sleep 
in the darkness of the ob serving-room before commencing 
operations. 
The greatest care should be taken in focussing the tele- 
scope. When high powers are used, this is a matter of some 
delicacy. It would be well if the eye-pieces intended for a 
telescope were so constructed that when the telescope is 
