Encyclopaedia Britannica > Volume 15, NIC-PAR

(I P T
274 . .
Optical In- With regard to the tlouhfe reflecting microscope,
strumcnts. Mr Baker observes, that the power ot the object-lens
v is indeed greatly increased by the addition ot two eye
glasses but as no object-lens can be used with them
of so minute a diameter, or which magnifies of itselt
near so much as those that can be used alone, the
glasses of this microscope, upon the whole, magnily
little or nothing more than those of Mr Wilson’s single
one *, the chief advantage arising from a combination
of lenses being the sight of a larger portion of the ob¬
ject.
Sect. V. Telescopes.
I. The Refracting Telescope.
Of the a- 1. T/ie Astronomical Telescope.—From what has been
stronomical said concerning the compound microscope, the nature of
telescope. tlie common astronomical telescope will easily be. under¬
stood : for it differs from the microscope only in this,
that the object is placed at so great a distance from it,
that the rays of the same pencil flowing from the object,
may be considered as falling parallel upon the object-
glass } and therefore the image made by that lens is con-
pi, sidered as coincident with its focus of parallel rays.
ccclxxKvii. I. This will appear very plain from fig. 4. in which
fig. 12. AB is the object emitting the several pencils of rays
A c of, B c d, &c. but supposed to be at so great a di¬
stance from the object-glass, c d, that the rays of the
same pencil may be considered as parallel to each other j
they are therefore supposed to be collected into their
respective foci at the points m and p, situated at the fo-
' cal distance of the object-glass c d. Here they form an
image E, and crossing each other proceed diverging to
the eye-glass hg ; which being placed at its own focal
distance from the points m and p, the rays of each pen¬
cil, after passing through that glass, will become paral¬
lel among themselves but the pencils themselves will
converge considerably with respect to one another, even
so as to cross at e, very little farther from the glass g h
than its focus 5 because, when they entered the glass,
their axes were almost parallel, as coming through the
object-glass at the point k, to whose distance the breadth
of the eye-glass in a long telescope bears very small pro¬
portion. So that the place of the eye will be nearly at
the focal distance of the eye-glass, and the rays of each
respective pencil being parallel among themselves, and
their axes crossing each other in a larger angle than
they would do if the object were to be seen by the na¬
ked eye, vision will be distinct, and the object will ap-
„ pear magnified.
/ts magni- The magnifying power in this telescope is as the fo-
iyingpower, cal length of the object-glass to the focal length of the
eye-glass.
In order to prove this, we may consider the angle
A A: B as that under which the object would be seen by
the naked eye •, for in considering the distance of the
object, the length of the telescope may be omitted, as
bearing no proportion to it. Now the angle under
which the object is seen by means of the telescope is
g e h, which is to the other A A: B, or its equal g k h, as
the distance from the centre of the object-irlass to that
of the eye-glass. The angle, therefore, which an ob¬
ject subtends to an eye assisted by a telescope of this
kind, is to that under which it subtends to the naked
3
I CS. PartlH
eve as the focal length of the object-glass to the focal Optical Ir-
length of the ^ye-glass. -
It is evident from the figure, that the visible area,
or space which can be seen at one view, when we look
through this telescope, depends on the breadth ot the
eye-glass, and not of the object-glass ; tor it the eye¬
glass be too small to receive the rays g m, p n, the ex ¬
tremities of the object could not have been seen at all :
a larger breadth of the object-lass conduces only to the
rendering each point of tbe image more luminous, by
receiving a larger pencil ot rays trom each point ot the
object. . 2Sl
It is in this telescope as in the compound microscope, Objects ,
where we see not the object itself, but only its •e™tl“
CED : now that image being inverted with respect to • |
the object, because the axis of the pencils that flow
from the object cross each other at kf objects seen
through a telescope of this kind necessarily appear in-
verted.
This is a circumstance not at all regarded by astro¬
nomers : but for viewing objects upon the earth, it is
convenient that the instrument should represent them m Plate
their natural posture ; to which use the telescope with cccW
three eye-glasses, as represented fig. 13. is peculiarly 0
adapted. 1 -t
AB is the object sending out the several pencils
A c d, B c d, &c. which passing through the object-
glass c d, are collected into their respective foci in CD, 25: |
where they form an inverted image. From this they Common
proceed to the first eye-glass ef, whose focus being atje^,
/, the rays of each pencil are rendered parallel among s])(ws obJ
themselves, and their axes, which were nearly paralleljects eret
before, are made to converge and cross each other:
the second eye-glass g Zi, being so placed that its focus
shall fall upon m, renders the axes of the pencils which
diverge from thence parallel, and causes the rays of
each, which were parallel among themselves, to meet
again at its focus EF on the other side, where they
form a second image inverted with respect to the for¬
mer, but erect with respect to the object. Now this
image being seen by the eye at a b through the eye¬
glass i k, affords a direct representation of the object,
and under the same angle that the first image CD
■would have appeared, had the eye been placed at /, sup¬
posing the eye-glasses to be ot equal convexity j and
therefore the object is seen equally magnified in this as
in the former telescope, that is, as the focal distance of
the object-glass to that of any one of the-glasses, and
appears erect. 253
2. The Galilean Telescope with the concave eye-glass Galilean
. 1 r n telescope,
is constructed as follows. plate
AB is an object sending forth the pencils of ray9 ccciXXxvih
g hi, kl m, &c. which, after passing through the ob- fig. 1.
ject-glass c d, tend towards eY.J (where we shall sup¬
pose "the focus of it to be), in order to form an inverted
image there as before j but in their way to it are made
to pass through the concave glass n 0, so placed that its
focus may fall upon E, and consequently the rays of
the several pencils which were converging towards those
respective focal points <?, E,/, will be rendered paralle],
but the axes of those pencils crossing each other at I ,
and diverging from thence, will be rendered more di¬
verging, as represented in the figure. Now these rays
entering the pupil of an eye, will form a large and di¬
stinct image a b upon the retina, which will be inverted