TELESCOPE, 
a great part of the heavens may be seen at 
once. In the inside of its tube, and ex- 
actly at the focus of the eye glass, tliere 
are two slender wires, which cros^ each 
other in the axis of the telescopei Now 
the finder is adjusted by means of screws 
upon the tube of the great telescope, in 
such a manner as that when an object, seen 
through the finder, appears to be near the 
crossing of the above-mentioned wires, it is 
at the same time visible through the great 
telescope: hence, when the observer wishes 
to view a small distant object, as a star, a 
planet, &c. he moves the instrument to one 
side or tbe other, until, by looking through 
the finder, he brings the object nearly to 
coincide with the crossing of the wires, 
and when that takes place, he immediately 
looks through the large telescope, &c. 
A micrometer is an instrument, which is 
used with a telescope, for the purpose of 
measuring small angles. A great variety of 
micrometers have been contrived by va- 
rious ingenious persons ; and they are more 
or less complicated, more or less expensive, 
as also more or less accurate. See Micro- 
meter. 
“ Achromatic Telescope,” is a name given 
t6 the refracting telescope, invented by Mr. 
John Dollond, and so contrived as to re- 
medy the aberration arising from colours, 
or the different l efrangibility of the rays of 
liaht. The improvement made by Mr. 
Dollond in his telescopes, by making two 
object-glasses of crown-glass, and one of 
flint which was tried with success when 
concave eye-glasses were used, was com- 
pleted by his son Peter Dollond; who, 
conceiving that the same method might be 
practised with success with convex eye- 
glasses, found, after a few trials, that it 
might be done. Accordingly he finished 
an object-glass of five feet focal length, 
with an aperture of 3| inches, composed of 
two convex lenses of crown-glass, and one 
concave of white flint glass. But appre- 
hending afterward that the apertures might 
be admitted still larger, he completed one 
of 3i feet focal length, with tlie same aper- 
ture of 3| inches. In tlie 17-inch improved 
achromatic telescope, the object-glass is 
composed of tliree glasses, viz, two convex 
of crown-glass, and one concave of white 
flint-glass: tlie focal distance of this com- 
bined object-glass is about seventeen inches, 
and the diameter of the aperture two in- 
ches. There are four eye-glasses contained 
in the tube, to be used for land objects ; 
the magnifying power with these is near 
fifty times; and they are adjusted to dif- 
ferent sights, and to different distances of 
the object, by turning a finger screw at the 
end of the outer tube. Tiiere is another 
tube, containing two eye glasses that mag- 
nify about seventy times, for hstronomical 
purposes. The telescope may be directed 
to any object by turning two screws in the 
stand on which it is fixed, the one giving a 
vertical motion, and the other a horizontal 
one. The stand may be inclosed in the in- 
side of tbe brass tube. 
The object-glass of the and 3i feet te- 
lescopes is composed of two glasses, one 
convex of crown-glass, and the other con- 
cave of white flint glass ; and the diameters 
of their apertures are two inches and 2| in- 
ches. Each of them is furnished with two 
tubes; one for land objects, containing four 
eye-glasses, and another with two eye- 
glasses for astronomical uses. They are 
adjusted by buttons on the outside of the 
wooden tube : and the vertical and horizon- 
tal motions are given by joints in the stands. 
The magnifying power of the least of these 
telescopes, with the eye-glass for land ob- 
jects, is nearly fifty times, and with those for 
astronomical purposes, eighty times ; and 
that of the greatest for land objects is nearly 
seventy times, but for astronomical obser- 
vations eighty and a hundred and thirty time.s ; 
for this lias two tubes, either of which may 
be used as occasion requires. Tliis tele- 
scope is also moved by a screw and rack- 
work, and the screw is turned by means of 
a hook’s joint. 
We must now say something of the spe- 
cula of telescopes, having referred to this 
place from the article Speculum. The 
metals of reflecting telescopes are generally 
composed of thirty-two parts of copper and 
fifteen of grain tin, with tiie addition of two 
parts of arsenic, to render the composition 
more white and compact. It has been ascer- 
tained, by a variety of experiments, that if 
one part qf brass, and one of silver, be added 
to this composition, and only one of arsenic 
used, a most excellent metal will be ob- 
tained, which is tlie whitest, hardest, and 
most reflective. The first composition is, 
however, for inexperienced persons, the 
best, ,as the easiest to cast, to grind, and 
polish. When this is employed, the cop- 
per and tin should be melted, and when 
mixed together should be poured into cold 
water, wiiich will separate the mass into a 
number of small particles. These small 
pieces of metal are then to be collected and 
put into the crucible, along with the silver 
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