THE MICROSCOPE, THE TELESCOPE, AND THE MAGIC-LANTERN.
The simple microscope--Use of the simple microscope in the
telescope--The terrestrial telescope--The Galilean telescope--The
prismatic telescope--The reflecting telescope--The parabolic
mirror--The compound microscope--The magic-lantern--The
bioscope--The plane mirror.
In Fig. 119 is represented an eye looking at a vase, three inches high,
situated at A, a foot away. If we were to place another vase, B, six
inches high, at a distance of two feet; or C, nine inches high, at three
feet; or D, a foot high, at four feet, the image on the retina would in
every case be of the same size as that cast by A. We can therefore lay
down the rule that _the apparent size of an object depends on the angle
that it subtends at the eye_.
[Illustration: FIG. 119.]
To see a thing more plainly, we go nearer to it; and if it be very
small, we hold it close to the eye. There is, however, a limit to the
nearness to which it can be brought with advantage. The normal eye is
unable to adapt its focus to an object less than about ten inches away,
termed the "least distance of distinct vision."
THE SIMPLE MICROSCOPE.
[Illustration: FIG. 120.]
A magnifying glass comes in useful when we want to examine an object
very closely. The glass is a lens of short focus, held at a distance
somewhat less than its principal focal length, F (see Fig. 120), from
the object. The rays from the head and tip of the pin which enter the
eye are denoted by continuous lines. As they are deflected by the glass
the eye gets the _impression_ that a much longer pin is situated a
considerable distance behind the real object in the plane in which the
refracted rays would meet if produced backwards (shown by the dotted
lines). The effect of the glass, practically, is to remove it (the
object) to beyond the least distance of distinct vision, and at the same
time to retain undiminished the angle it subtends at the eye, or, what
amounts to the same thing, the actual size of the image formed on the
retina.[22] It follows, therefore, that if a lens be of such short focus
that it allows us to see an object clearly at a distance of two
inches--that is, one-fifth of the least distance of distinct vision--we
shall get an image on the retina five times larger in diameter than
would be possible without the lens.
The two simple diagrams (Figs. 121 and 122) show why the image to be
magnified should be nearer to the lens than the principal focus, F. We
have already seen (Fig. 109) that rays coming from a point in the
principal focal plane emerge as a parallel pencil. These the eye can
bring to a focus, because it normally has a curvature for focussing
parallel rays. But, owing to the power of "accommodation," it can also
focus _diverging_ rays (Fig. 121), the eye lens thickening the necessary
amount, and we therefore put our magnifying glass a bit nearer than F to
get full advantage of proximity. If we had the object _outside_ the
principal focus, as in Fig. 122, the rays from it would converge, and
these could not be gathered to a sharp point by the eye lens, as it
cannot _flatten_ more than is required for focussing parallel rays.
[Illustration: FIG. 121.]
[Illustration: FIG. 122.]
USE OF THE SIMPLE MICROSCOPE IN THE TELESCOPE.
[Illustration: FIG. 123.]
Let us now turn to Fig. 123. At A is a distant object, say, a hundred
yards away. B is a double convex lens, which has a focal length of
twenty inches. We may suppose that it is a lens in a camera. An inverted
image of the object is cast by the lens at C. If the eye were placed at
C, it would distinguish nothing. But if withdrawn to D, the least
distance of distinct vision,[23] behind C, the image is seen clearly.
That the image really is at C is proved by letting down the focussing
screen, which at once catches it. Now, as the focus of the lens is twice
_d_, the image will be twice as large as the object would appear if
viewed directly without the lens. We may put this into a very simple
formula:--
Magnification = focal length of lens
--------------------
_d_
[Illustration: FIG. 124.]
In Fig. 124 we have interposed between the eye and the object a small
magnifying glass of 2-1/2-inch focus, so that the eye can now clearly
see the image when one-quarter _d_ away from it. B already magnifies the
image twice; the eye-piece again magnifies it four times; so that the
total magnification is 2 × 4 = 8 times. This result is arrived at
quickly by dividing the focus of B (which corresponds to the
object-glass of a telescope) by the focus of the eye-piece, thus:--
20
____ = 8
2-1/2
The ordinary astronomical telescope has a very long focus object-glass
at one end of the tube, and a very short focus eye-piece at the other.
To see an object clearly one merely has to push in or pull out the
eye-piece until its focus exactly corresponds with that of the
object-glass.
THE TERRESTRIAL TELESCOPE.
An astronomical telescope inverts images. This inversion is inconvenient
for other purposes. So the terrestrial telescope (such as is commonly
used by sailors) has an eye-piece compounded of four convex lenses which
erect as well as magnify the image. Fig. 125 shows the simplest form of
compound erecting eye-piece.
[Illustration: FIG. 125.]
THE GALILEAN TELESCOPE.
[Illustration: FIG. 126.]
A third form of telescope is that invented by the great Italian
astronomer, Galileo,[24] in 1609. Its principle is shown in Fig. 126.
The rays transmitted by the object-glass are caught, _before_ coming to
a focus, on a concave lens which separates them so that they appear to
meet in the paths of convergence denoted by the dotted lines. The image
is erect. Opera-glasses are constructed on the Galilean principle.
THE PRISMATIC TELESCOPE.
In order to be able to use a long-focus object-glass without a long
focussing-tube, a system of glass reflecting prisms is sometimes
employed, as in Fig. 127. A ray passing through the object-glass is
reflected from one posterior surface of prism A on to the other
posterior surface, and by it out through the front on to a second prism
arranged at right angles to it, which passes the ray on to the compound
eye-piece. The distance between object-glass and eye-piece is thus
practically trebled. The best-known prismatic telescopes are the Zeiss
field-glasses.
[Illustration: FIG. 127.]
THE REFLECTING TELESCOPE.
We must not omit reference to the _reflecting_ telescope, so largely
used by astronomers. The front end of the telescope is open, there being
no object-glass. Rays from the object fall on a parabolic mirror
situated in the rear end of the tube. This reflects them forwards to a
focus. In the Newtonian reflector a plane mirror or prism is situated in
the axis of the tube, at the focus, to reflect the rays through an
eye-piece projecting through the side of the tube. Herschel's form of
reflector has the mirror set at an angle to the axis, so that the rays
are reflected direct into an eye-piece pointing through the side of the
tube towards the mirror.
THE PARABOLIC MIRROR.
This mirror (Fig. 128) is of such a shape that all rays parallel to the
axis are reflected to a common point. In the marine searchlight a
powerful arc lamp is arranged with the arc at the focus of a parabolic
reflector, which sends all reflected light forward in a pencil of
parallel rays. The most powerful searchlight in existence gives a light
equal to that of 350 million candles.
[Illustration: FIG. 128.--A parabolic reflector.]
THE COMPOUND MICROSCOPE.
We have already observed (Fig. 110) that the nearer an object
approaches a lens the further off behind it is the real image formed,
until the object has reached the focal distance, when no image at all is
cast, as it is an infinite distance behind the lens. We will assume that
a certain lens has a focus of six inches. We place a lighted candle four
feet in front of it, and find that a _sharp_ diminished image is cast on
a ground-glass screen held seven inches behind it. If we now exchange
the positions of the candle and the screen, we shall get an enlarged
image of the candle. This is a simple demonstration of the law of
_conjugate foci_--namely, that the distance between the lens and an
object on one side and that between the lens and the corresponding image
on the other bear a definite relation to each other; and an object
placed at either focus will cast an image at the other. Whether the
image is larger or smaller than the object depends on which focus it
occupies. In the case of the object-glass of a telescope the image was
at what we may call the _short_ focus.
[Illustration: FIG. 129.--Diagram to explain the compound microscope.]
Now, a compound microscope is practically a telescope with the object at
the _long_ focus, very close to a short-focus lens. A greatly enlarged
image is thrown (see Fig. 129) at the conjugate focus, and this is
caught and still further magnified by the eye-piece. We may add that the
object-glass, or _objective_, of a microscope is usually compounded of
several lenses, as is also the eye-piece.
THE MAGIC-LANTERN.
The most essential features of a magic-lantern are:--(1) The _source of
light_; (2) the _condenser_ for concentrating the light rays on to the
slide; (3) the _lens_ for projecting a magnified image on to a screen.
Fig. 130 shows these diagrammatically. The _illuminant_ is most commonly
an oil-lamp, or an acetylene gas jet, or a cylinder of lime heated to
intense luminosity by an oxy-hydrogen flame. The natural combustion of
hydrogen is attended by a great heat, and when the supply of oxygen is
artificially increased the temperature of the flame rises enormously.
The nozzle of an oxy-hydrogen jet has an interior pipe connected with
the cylinder holding one gas, and an exterior, and somewhat larger, pipe
leading from that containing the other, the two being arranged
concentrically at the nozzle. By means of valves the proportions of the
gases can be regulated to give the best results.
[Illustration: FIG. 130.--Sketch of the elements of a magic-lantern.]
The _condenser_ is set somewhat further from the illuminant than the
principal focal length of the lenses, so that the rays falling on them
are bent inwards, or to the slide.
The _objective_, or object lens, stands in front of the slide. Its
position is adjustable by means of a rack and a draw-tube. The nearer it
is brought to the slide the further away is the conjugate focus (see p.
239), and consequently the image. The exhibitor first sets up his screen
and lantern, and then finds the conjugate foci of slide and image by
racking the lens in or out.
If a very short focus objective be used, subjects of microscopic
proportions can be projected on the screen enormously magnified. During
the siege of Paris in 1870-71 the Parisians established a balloon and
pigeon post to carry letters which had been copied in a minute size by
photography. These copies could be enclosed in a quill and attached to a
pigeon's wing. On receipt, the copies were placed in a special lantern
and thrown as large writing on the screen. Micro-photography has since
then made great strides, and is now widely used for scientific purposes,
one of the most important being the study of the crystalline formations
of metals under different conditions.
THE BIOSCOPE.
"Living pictures" are the most recent improvement in magic-lantern
entertainments. The negatives from which the lantern films are printed
are made by passing a ribbon of sensitized celluloid through a special
form of camera, which feeds the ribbon past the lens in a series of
jerks, an exposure being made automatically by a revolving shutter
during each rest. The positive film is placed in a lantern, and the
intermittent movement is repeated; but now the source of illumination is
behind the film, and light passes outwards through the shutter to the
screen. In the Urban bioscope the film travels at the rate of fifteen
miles an hour, upwards of one hundred exposures being made every second.
The impression of continuous movement arises from the fact that the eye
cannot get rid of a visual impression in less than one-tenth of a
second. So that if a series of impressions follow one another more
rapidly than the eye can rid itself of them the impressions will
overlap, and give one of _motion_, if the position of some of the
objects, or parts of the objects, varies slightly in each succeeding
picture.[25]
THE PLANE MIRROR.
[Illustration: FIG. 131.]
This chapter may conclude with a glance at the common looking-glass. Why
do we see a reflection in it? The answer is given graphically by Fig.
131. Two rays, A _b_, A _c_, from a point A strike the mirror M at the
points _b_ and _c_. Lines _b_ N, _c_ O, drawn from these points
perpendicular to the mirror are called their _normals_. The angles A
_b_ N, A _c_ O are the _angles of incidence_ of rays A _b_, A _c_. The
paths which the rays take after reflection must make angles with _b_ N
and _c_ O respectively equal to A _b_ N, A _c_ O. These are the _angles
of reflection_. If the eye is so situated that the rays enter it as in
our illustration, an image of the point A is seen at the point A^1, in
which the lines D _b_, E _c_ meet when produced backwards.
[Illustration: FIG. 132.]
When the vertical mirror is replaced by a horizontal reflecting surface,
such as a pond (Fig. 132), the same thing happens. The point at which
the ray from the reflection of the spire's tip to the eye appears to
pass through the surface of the water must be so situated that if a line
were drawn perpendicular to it from the surface the angles made by lines
drawn from the real spire tip and from the observer's eye to the base of
the perpendicular would be equal.
[22] Glazebrook, "Light," p. 157.
[23] Glazebrook, "Light," p. 157.
[24] Galileo was severely censured and imprisoned for daring to maintain
that the earth moved round the sun, and revolved on its axis.
[25] For a full account of Animated Pictures the reader might
advantageously consult "The Romance of Modern Invention," pp. 166 foll.