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TELESCOPE. 
tial, or astronomical telescope to which presentation of terrestrial objects through 
may ne acidea^ tlie “ lialilean, or JDutch te- 
lescope,” the “ reflecting telescope the 
“ achromatic telescope,’’ &c. 
We shah proceed to describe some of 
these, in order to illustrate the principle. 
The “ astronomical telescope ” consists of 
two convex lenses, A B, K M, Plate XVI. 
Miscel. fig. 1, fixed at the two extremities ol 
a tube, that consists, at least, of two parts, 
that slide one within the other, for adjust- 
ing the focus in proportion to the distance 
of the objects that are to be seen through 
the telescope. 
PQ represents the semi-diameter of a 
very distant object, from every point of 
'Which rays come, so very little diverging to 
the object lens, K M, of the telescope, as to 
be nearly parallel : p f/ is the picture of the 
object, P Q, which would be formed upon 
a screen situated at that place. Beyond 
that place, the rays of every single radiant 
point proceed divergingly upon another 
lens, A B, called tlie eye-glass, which is 
wore convex than the former, and are, hy 
this, caused to proceed parallel to one ano- 
ther, in which direction they enter the eye 
of the observer at O. 
The two lenses of this telescope have a 
common axis, OLQ; L5 is the focal dis- 
tance of the object lens, and E q is the focal 
distance of the eye lens. An object view- 
ed through this telescope, by an eye si- 
tuated at O, will appear distinct, inverted, 
and magnified ; viz. the object seen with- 
out the telescope will be, to its appearance 
through the telescope, as qE to 5L; that 
is, as the focal distance of the eye lens to 
the focal distance of the object lens. For 
the rays, see Optics, which, after their 
crossing at the place, rqp, proceed diverg- 
ingly, fall Upon the lens, AB, in the same 
manner as if a real object were situated at 
rqpi and of course, on the other side of 
that lens the rays of each pencil will pro- 
ceed parallel. Now to the eye at O, the 
apparent magnitude of the object, or of the 
part, PQ, is measured by the angle, E O A, 
or by its equal, q Ep; but to the naked eye 
at Ly when the glass is removed, the appa- 
rent magnitude of the object is measured 
by the angle, Q E P, or by its equal, qEp- 
therefore the apparent magnitude, to the 
naked eye, is to the apparent magnitude 
through the telescope, as the angle, jLp, 
is to the angle, qEp; or as the distance^ 
q E, is to the distance, q L. This telescope 
is mostly used for astronomical observa- 
tions; for, as it inverts the object, there- 
It would not be pleasant. It is evident, 
from the above explanation, that if the two 
lenses of this telescope have equal focal dis- 
tances, the telescope w’ill not magnify. It 
also appears, that, with a given object lens, 
the shorter the focus of the eye lens is, the 
greater will the magnifying power be. But 
when the disproportion of the two focal 
lengths is very great, then the aberration, 
arising from the figure of the lenses, and 
from the dispersive power of glass, becomes 
so very great as to do more damage than 
can be compensated by the increased mag- 
nifying power. Hence, in order to obtain 
a very great magnifying power, those tele- 
scopes have sometimes been made very 
long, as, for instance, of 100 feet, or up- 
wards ; and as they were used for astrono- 
mical purposes, or mostly in the night time, 
they were frequently used without a tube, 
viz. fhfe object lens was fixed on the top of 
a pole, in a frame capable of motion in any 
required direction, and the eye lens was fix- 
ed in a short tube that was held in the hand 
of the observer. The distance, as well as 
the direction, of the two lenses, was adjust- 
ed by a strong cord stretched between the 
frame of the object lens and the tube of the 
eye lens. In this construction, the instru- 
ment has been called an “ aerial telescope.” 
Its use is evidently incommodious ; but it 
was w'ilh such a telescope that five satellites 
of Saturn, and other remarkable objects, 
were discovered. 
The object, which appears inverted 
through this telescope, will appear upright 
and distinct if two more convex eye glasses 
be subjoined to it, at a distance from each 
other, which is equal to the sum of their fo- 
cal distances ; and when their local dis- 
tances are equal, the object will be magni- 
lied as much as without those additional 
glasses ; but through them it w'ill appear 
upright, and not inverted. Hence this te- 
lescope has been mostly used for viewing 
terrestrial objects, and is therefore called 
the “ terrestrial telescope.” 
The “ Galilean telescope ” consists of a 
convex object lens, and a concave eye lens, 
and derives its name from the great Gali- 
leo, who is generally reckoned the inventor 
of It. Fig. 2 shows, that the distance be- 
tween the two lenses is less than tiie focal 
distance of the object lens ; viz. instead of 
the convex lens situated behind the place 
of the image, to make the rays of each pen- 
cil proceed in a parallel direction to the 
eye, here a concave eye lens is placed as 
B b 2 
