TELESCOPE. 
much before that image ; and this lens opens 
the rays of each pencil that converged to q 
and p, and makes them emerge parallel to- 
wards the. eye ; as is evident Ky conceiving 
the rays to go back again through the eye 
lens, whose focal distance is l^q- 
The eye must be placed close to the con- 
cave lens,jn order to receive as many pen- 
cils as possible; and then supposing an 
emerging ray of an oblique pencil to be 
produced backwards along A O, the appa 
rent magnitude of the object is measured 
by the angle, AOE, or its equal gEp, 
wliicli is to the angle, gLp, or Q El , as g 
to gE, viz. as in the astronomical telescope. 
It is evident, that in this telescope the ob- 
jects appear erect, for the rays .of light do 
not cross each other. 
The field of view, or quantity of objects 
that are taken in at once in this telescope, 
does not depend upon the 
eyelens,asin the astronomical te escope, 
but upon the breadth of the pupil of the 
eye ; because the pupil is less than the ey 
lens, A B, and the lateral pencils do not 
now converge to, but diverge from, the axis 
of the lenses. Upon this account the view 
is narrower in this than in the preceding 
telescope; vet the objects through it ap- 
pear remarkably clear and distinct. 
“The night telescope” is a short tele- 
scope, viz. about two feet long, which re- 
presents the objects inverted, much en- 
lightened, but not much magmhed. Its 
field of view is also very extensive. This 
telescope, in consequence of those proper- 
ties, is used at night mostly by navigators, 
for the purpose of discovering objects that 
are not very distant, but which cannot 
otherwise be seen, for want of sufficient 
. light ; such as vessels, coasts, rocks, &c. 
On account of its extensive field, and great 
lioht, this telescope has also been advan- 
tageously used, by astronomers, for disco- 
■yering some celestial objects, whose sitiia- 
tion was not exactly known, or for viewing 
at once the relative situation of several stars 
and other objects. 
This telescope has a pretty large and sim- 
ple object lens, whence it derives its great 
light ; for as the rays which proceed from 
every single point of the object fall upon 
the whole lens of a telescope, and are 
tlience refracteid to a focus, it is evident 
that the larger tliht lens is, the greater num- 
ber of rays \yill be thrown upon that focus, 
and of course the brighter will the image 
be. In this telescope latge lens may be 
ased, because the telescope is not intended 
to magnify mere than about four oi‘ si* 
times in lineal extension. 
Within this telescope a second lens is of- 
ten used for shortening the focal length of 
the object lens. The eye lens is sometimes 
single, blit mostly double, {viz. a combina- 
tion of two plano-convex lenses placed at a 
little distance from each other) and preily 
large : hence is derived the extensive field 
of view, which in some of those telescopes 
exceeds six or seven degrees. 
AEe may observe, once for all, that in 
every telescope the distance between the 
object lens and the other lens or lenses must 
be alterable, in order that the focus may be 
adjusted according to tlie distance of the 
objects. Hence, every telescope consists 
at least of two tubes, one of which, viz. 
that with the eye lenses, slides within the 
other. To the .same telescope several eye 
tubes, with a shallower or deeper lens, or 
with a different number of lenses, may be 
adapted successively, in order to give them 
different magnifying powers, suitably to the 
clearness of the air, of the objects, &c. as 
also for converting them info astronomical 
or terrestrial telescopes. 
We now proceed to the reflected tele- 
scope, which is likewise called the Newto- 
nian telescoipe, for if not the original pro- 
jector, Sir Isaac Newton is, at least, the^ 
first person who executed a telescope of 
this sort, which consists of reflecting and 
refracting parts. 
The general principle of this telescope is 
the same as that of the djoptric or refract- 
ing telescope. In the latter the rays which 
come from a distant object are, by the action 
of the donvex object lens, collected to a fo- 
cus, and beyond that focus the rays of every 
single radiant point are rendered again pa- 
rallel by the action of the eye lens or eye 
lehses. This is otherwise expressed, by 
saying that the object lens forms an image 
of the object, which image is viewed by the 
eye lens. In the former, viz. in the reflect- 
ing telescope, the rays which come from a 
distant object, are, by the action of a con- 
cave reflector, sent back convergingly to U 
focus, wliere they form an image, which is 
viewed through the eye lens. There are 
several varieties of this telescope ; we shall 
content ourselves with the description of 
one only, viz. the Gregorian telescope, 
which is represented in fig. 3. The large 
concave speculum, B E, of this telescope is 
perforated with a hole quite through its 
middle. Witliin the tube of the telescope 
a small concave speculum, x y, is supported 
