THE TELEPOLARISCOPE.
In previous chapters we have considered the lessons that we can learn
from light—from the vibrations of the so-called ether—when we put
questions to it through various instruments as interpreters. There is
still another method of putting questions to these same vibrations, and
the instrument we have now to consider is the Polariscope.
The spectroscope helped us to inquire into the lengths of the
luminiferous waves; from the polariscope we learn whether there is any
special plane in which these waves have their motion.
The polariscope is an instrument which of late years has become a useful
adjunct to the telescope in examining the light from a body in order to
decide whether it is reflected or not, and to ascertain indirectly the
plane in which the rays reflected to the eye lie. The action of the
instrument depends upon the fact that light which consists solely of
vibrations perpendicular to a given plane is said to be completely
polarized in that plane. Light that contains an excess of vibrations
perpendicular to a given plane is said to be partially polarized in that
plane.
It was Huyghens that discovered the action of Iceland spar in doubly
refracting light; and the light which passed the crystal was called
_polarized light_ at the suggestion of Newton, who, it must be
remembered, looked upon light as something actually emitted from
luminous bodies; these projected particles were supposed, after passage
through Iceland spar, to be furnished with poles analogous to the poles
of a magnet, and to be unable to pass through certain bodies when the
poles were not pointing in a certain direction. It was not until the
year 1808 that Malus discovered the phenomenon of polarization by
reflection. He was looking through a double-refracting prism at the
windows of the Luxembourg Palace, on which were falling the rays of the
setting sun. On turning the prism he noticed the ordinary and
extraordinary images alternately become bright and dark. This phenomenon
he at once saw was in close analogy to that which is observed when light
is passed through Iceland spar. At first he thought it was the air that
polarized the light, but subsequent experiments showed him that it was
due to reflection from the glass.
Let us examine some of the phenomena before we proceed to show the use
astronomers make of them.
It is the property of some crystals, such as tourmaline, when cut
parallel to a given direction, called the optic axis of the crystal, to
absorb all vibrations or resolved parts of vibrations perpendicular to
this line, transmitting only vibrations parallel to it.
A similar absorption of vibrations perpendicular to a given direction
may be effected by various other combinations, of which one, Nicol’s
prism, is in most common use. Any of these arrangements may be used as
an analyzer with the telescope, for determining whether the light is
completely or partially polarized, and in either of these cases which is
the plane of polarization. The plane containing the direction of the
rays and the line in the analyzer to which the transmitted vibrations
are parallel, is called the plane of analyzation: all the light which
reaches the eye consists of vibrations in the plane of analyzation. As
we rotate the analyzer, we rotate equally the plane of analyzation. If
we find a position of the plane of analyzation for which the light
received by the eye is a maximum, we know that the light from the object
is partially or completely polarized in a plane perpendicular to the
plane of analyzation when in this position. To determine whether the
polarization is partial or complete, we must turn the analyzer through
an angle of 90° from this position: if we now obtain complete darkness,
we know that there are no vibrations having a resolved part parallel to
the plane of analyzation in this position, or that the light is
completely polarized in this plane: if there be still some light
visible, the polarization is only partial.
To explain this a little more fully, we may compare the vibrations or
waves of light to waves of more material things: we may have the
vibrating particles of the ether moving up and down as the particles do
in the case of a wave of water, or the particles may move horizontally
as a snake does in moving along the ground. We may consider that
ordinary light consists of vibrations taking place in all planes, but if
it passes through or is reflected by certain substances at certain
angles, the vibrations in certain planes are, as it were, filtered out,
leaving only vibrations in a certain plane. This light is then said to
be polarized, and its plane of polarization is found by its power of
passing through polarizing bodies only when they are in certain
positions.
If, for instance, a ray of ordinary light is passed through a crystal of
tourmaline, the vibrations of the filtered ray will only lie in one
plane; if then a second crystal of tourmaline be held in a similar
position to the first, the ray will pass through it unaffected; but if
it be turned through a quarter of a circle about the ray as an axis, the
ray will no longer be able to pass, for being in a position at right
angles to the first, it will filter out just the rays that the first
allows to pass. For illustration, take a gridiron: if we attempt to pass
a number of sheets of paper held in all positions through it, only those
in a certain plane, viz., that of the rods forming the gridiron, could
be passed through, and those that would go through would also go through
any number of gridirons held in a similar position. But if another
gridiron be placed so that its bars cross those of the first, the sheets
of paper could no longer pass, and it is evident that if we could not
see or feel the paper, we could tell in what plane it was by the
position in which the gridiron must be held to let it pass, and having
found the paper to be, say horizontal, we know that the bars of the
first gridiron are also horizontal. So with light, we can analyze a ray
of polarized light and say in what plane it is polarized.
The example of the gridiron, however, does not quite represent the
action of the second crystal; for if the bars of the second gridiron are
turned a very small distance out of coincidence with those of the first,
the sheets of paper would be stopped; but with light, the intensity of
the ray is only gradually diminished, until it is finally quenched when
the axes of the crystals are at right angles to each other.
[Illustration:
FIG. 204.—Diagram showing the Path of the Ordinary and Extraordinary
Ray in Crystals of Iceland Spar.
]
Light is polarized by transmission and by reflection. We have already,
when we were discussing the principle involved in the double-image
micrometer, seen how a crystal of Iceland spar divides a ray into two
parts at the point of incidence. Now these two rays are _oppositely
polarized_, that is to say, the vibrations take place in planes
perpendicular to each other; the vibrations of the incident light in one
plane are refracted more than the vibrations in the opposite plane, and
we have therefore two rays, one called the ordinary ray, and the other
the extraordinary ray. Fig. 204 shows a ray of light, S I, incident on
the first crystal at I; it is then divided up into the ordinary ray I R
and the extraordinary one I R´; a screen is then interposed, stopping
the extraordinary ray and allowing the ordinary one to fall on the
second crystal at I. If then this crystal be in a similar position to
the first, this ray, vibrating only in one plane, will pass onwards as
an ordinary ray, I R; there being no vibrations in the perpendicular
plane to form an extraordinary ray, there will be only one circle of
light thrown on the screen at O by the lens. But, if the second crystal
be turned round the line S S as an axis, the plane of vibration of the
ray falling on its surface will no longer coincide with the plane in
which an ordinary ray vibrates in the crystal, and it therefore becomes
split up into two, one vibrating in the plane as an ordinary ray, and
the other in that of an extraordinary ray; we have therefore the ray I
R´ in addition to the first, and consequently a second circle on the
screen at E´. As the crystal rotates, the plane of extraordinary
refraction becomes more and more coincident with the plane of vibration
of the incident ray, until, when it has revolved through 90°, it
coincides with it exactly; it then passes through totally as an
extraordinary ray, and as the refractive power of the crystal is greater
for vibrations in this plane, we get all the light traversing the
direction I R and falling on the screen at E´, and there being then no
light ordinarily refracted, the circle O disappears. Fig. 205 shows the
relative brightness of the circles E and O as they revolve round the
centre S of the screen, the images produced by the ordinary and the
extraordinary ray becoming alternately bright and dark as the crystal is
rotated. Fig. 206 shows the images on the screen when the ordinary ray
is stopped by the first screen instead of the extraordinary one.
[Illustration:
FIG. 205.—Appearance of the Spots of Light on the Screen shown in the
preceding Figure, allowing the ordinary ray to pass and rotating the
second Crystal.
]
[Illustration:
FIG. 206.—Appearance of Spots of Light on Screen on rotating the
second Crystal, when the extraordinary ray is allowed to pass
through the first Screen.
]
A crystal of tourmaline acts in a like manner to Iceland spar, but the
ordinary ray is rapidly absorbed by the crystal, so that the
extraordinary ray only passes. There is an objection to the use of it,
as it is not very transparent, and a Nicol’s prism is now generally used
for polarizing light. It is constructed out of a rhombo-hedron of
Iceland spar cut into two parts in a plane passing through the obtuse
angles, and the two halves are then joined by Canada balsam. The
principle of construction is this: the power of refracting light
possessed by Canada balsam is less than that possessed by Iceland spar
for the ordinary ray, and greater in the case of the extraordinary ray;
in consequence, the ordinary ray is reflected at the surface of
junction, while the extraordinary ray passes onwards through the
crystal.
[Illustration:
FIG. 207.—Instrument for showing Polarization by Reflection.
]
It is manifest then that if two Nicols are used instead of two simple
crystals, represented in Fig. 204, there will be only one spot of light
on the screen, which is due to the extraordinary ray, and as in certain
positions this no longer passes (for the ordinary ray, which appears in
the place of the extraordinary when the crystal is used, cannot pass
through the Nicol), no light at all passes in such positions, so that we
can use the second Nicol as an analyzer to ascertain in what plane the
light is polarized.
Light is also polarized by reflection from the surface of a transparent
medium. When a ray of ordinary light falls on a plate of glass at an
angle of 54° 55´ with the normal, the reflected ray is perfectly
polarized, and at other inclinations the polarization is incomplete.
Here then is polarization by reflection. Fig. 207 shows an apparatus for
producing this phenomenon. The light foiling on the first mirror from E
is reflected through the tube as a polarized beam, and this is analyzed
by the other mirror (I), whose plane can be rotated round the axis of
the tube. The angle of polarization differs with different substances
according to their refractive power, for polarization of the reflected
ray is perfect only when the angle of incidence is such that the
reflected ray is at right angles to the refracted one.
As a result of what we have said, the light of the sun reflected from
the surface of water or from the glass of a window is polarized, and
although it may be dazzling to the eye, it is reduced, or even entirely
cut off, when falling at the polarizing angle, by looking through the
transparent Nicol’s prism or plate of glass held in certain positions
and acting as an analyzer. On rotating the analyzer there is an
alternation of intensity, and by looking at the window through a crystal
of Iceland spar as an analyzer, two images would be seen which would
alternate in brightness as the crystal is rotated. So also there is a
difference in the intensity of the light from the sky when the analyzer
is rotated, showing that the light reflected from the watery and dust
particles in the air is polarized, and by the position of the analyzer
we find that it is polarized in the plane we should expect if it be, as
it is, reflected from the sun.
* * * * *
It will be asked, however, what is the astronomical use of determining
whether light has an excess of vibrations in any given direction?
To this we may reply that light that is reflected from any body is
generally partially polarized in the plane of reflection, and that if we
find that the light received from any body is partially polarized in a
given plane, we may conclude that it has very likely been reflected in
that plane.
Hence then in the case of any celestial body the origin of the light of
which is doubtful, the polariscope tells us whether the light is
intrinsic or reflected.
It tells us more than this, it tells us the plane in which the
reflection has taken place. As the polarization takes place, when it
does take place, at the celestial body, all we have to do is to attach
an analyzer to the telescope.
A careful application of the above principles has shown that the light
from the sun’s corona is partially polarized, and in the same plane as
it would be if reflected from small particles in the neighbourhood of
the sun: so also a portion of the light of Coggia’s Comet was found to
be polarized, and therefore we say that it reflected sunlight in
addition to its own proper light.
In what has been hitherto said we have only considered the use of a
Nicol, or glass plates, or crystal of Iceland spar as an analyzer, and
by the variation of brightness the presence and plane of polarization
have been determined; but unless the polarization is somewhat decided,
it could not be detected by this method. Advantage is therefore taken of
the fact that a plate of quartz rotates the plane of polarization of a
ray passing through it, and it rotates the more refrangible colours more
than the others, and some crystals rotate the plane one way, and others
in the opposite direction: the crystals are therefore called
respectively right- and left-handed quartz; the thicker the quartz the
greater the angle through which the plane of polarization is twisted.
This supplies us with a most delicate apparatus, which we next describe.
A crystal of right- and a crystal of left-handed quartz are taken and
cut to such thickness that a ray of any colour, say green, has its plane
turned through 90° on passing through each of them. They are then cut
into the form of a semicircle and placed side by side. Any change of the
angle of polarization will now affect each plate differently. In one
plate the colours will change from red to violet, in the other from
violet to red.
If now a ray of polarized light, say vibrating in a vertical plane,
falls on them, the green rays will have their plane of vibration turned
through 90° by each crystal, and the vibration of the green from both
crystals will then be in the horizontal plane. Nicol’s prism interposed
between the quartz plates and the eye, so as to allow horizontal
vibrations to pass, will show the green from both crystals of equal
intensity; the rays of other colours, being turned through a greater or
less angle than 90°, will not be vibrating horizontally, and will
therefore only partially pass through, so green will be the prevailing
colour. If now the plane of vibration of the original ray be turned a
little out of the vertical, the ray, on the red side of the green, will
appear in one half, and that on the violet side of the green in the
other: so that immediately the plane of polarization changes, the plates
transmit a different colour, and the apparatus must be twisted round
through just the same angle as the polarized ray in order to get the
crystals of the same colour. It is therefore obvious that the angle made
by a polarized ray with a fixed plane is easily ascertained in this
manner.
There is also another instrument for detecting polarization which is
perhaps more commonly used than the biquartz: it is generally called
Savart’s analyser, and is extremely sensitive in its action. On looking
through it at any object emitting ordinary light, the white circle of
light limited by the aperture of the instrument only is seen; but if any
polarized light should happen to be present, a number of parallel bands,
each shaded from red to violet, make their appearance; on rotating the
instrument a point is found when a very slight motion causes the bands
to vanish and others to appear in the intermediate spaces, and knowing
the position required for the change of bands with light polarized in a
known plane, say the vertical plane, it is easy to find how far the
plane of polarization of any ray is from the vertical, by the number of
degrees through which the instrument must be turned to change the bands.
The construction of the instrument, and especially its action, is not
easy to understand without a considerable knowledge of optics, but it
may be stated that a plate of quartz is cut, in a direction inclined at
45° to its axis, into two parts of the same thickness; one part is then
turned through a right angle and placed with the same surfaces in
contact as before; these are fixed in the instrument so that the light
shall traverse them perpendicularly to the plane of section; the light
then passes through a Nicol’s prism as an analyser to the eye. The lines
observed, “black centred” in one position, and “white centred” in the
position at right angles to this, are always in the direction before
referred to. The delicacy of the test supplied by this arrangement
increases as this direction is more nearly parallel or perpendicular to
the plane of polarization of the ray under examination.