THE CHEMISTRY OF THE STARS (CONTINUED): THE TELESPECTROSCOPE.
We have now to speak of the methods of using these spectroscopes for the
purpose of astronomical observations. For a certain class of
observations of the sun no telescope is necessary, but some special
arrangements have to be made.
Thus while Dr. Wollaston and Fraunhofer were contented with simple
prisms, when Kirchhoff observed the solar spectrum, and made his careful
maps of the lines, he used an instrument like Fig. 173, and for the
purpose of comparing the spectrum of the sun with that of each of the
chemical elements in turn, he used a small reflecting prism, covering
one-half of the slit, Fig. 188, so that any light thrown sideways on to
the slit would be caught by this prism, and reflected on to the slit as
if it came from an object near the source of light at which the
spectroscope is pointing, so that one-half of the slit can be
illuminated by the sun, while the other is illuminated by another light;
and on looking through the eyepiece one sees the two spectra, one above
the other; so that we are able to compare the lines in the two spectra.
The sunlight, whether coming from the sun itself or a bright cloud, is
reflected, into the comparison prism, Fig. 189, of the spectroscope. An
instrument called a heliostat can be used for this, reflecting the light
either directly into the prism or through the medium of other
reflectors.
[Illustration:
FIG. 188.—Comparison Prism, showing the path of the Ray.
]
The heliostat is a mirror, mounted on an axis, which moves at the same
rate as the sun appears to travel, so that wherever the sun is, the
reflector, once adjusted, automatically throws the beam into the
instrument, so that the light of the moving sun can be observed without
moving the spectroscope.
[Illustration:
FIG. 189.—Comparison Prism fixed in the Slit.
]
An average solar spectrum is thus obtained, and, by means of a prism
over one-half of the slit, it was quite possible for Kirchhoff and
Bunsen to throw in a spectrum from any other source for comparison, and
so they compared the spectra of the metals and other elements with the
solar spectrum, and tested every line they could find in the spectra.
They first found that the two lines of sodium corresponded with the two
lines called D in the spectrum, then that the 460 lines of iron
corresponded in the main with dark lines in the solar spectrum; and so
they went on.
[Illustration:
FIG. 190.—Foucault’s Heliostat.
]
There is, however, a method of varying the attack on this body
altogether, by means of the spectroscope and telescope. We saw that
Kirchhoff and Bunsen contented themselves with an average spectrum of
the sun—that is to say, they dealt with the general spectrum which they
got from the general surface of the sun, or reflected from a cloud or
any other portion of the sky to which they might direct the reflector;
but by means of some such an arrangement as is shown in Fig. 192, we can
arrange our spectroscope so that we shall be able to form _an image_ of
the sun by the object-glass of a telescope, on the slit, and allow it to
be immersed in any portion of the sun’s image we may choose. We then
have a delicate means of testing what are the spectroscopic conditions
of the spots and of those brighter portions of the sun which are called
faculæ, and the like. And it is known that, by an arrangement of this
kind, it is even possible to pick up, without an eclipse, those strange
things which are called the red prominences, or the red flames, which
have been seen from time to time during eclipses.
If we wish to observe any of the other celestial bodies, we must employ
a telescope and form an image on the slit, or else use the heavenly body
itself as a slit. In the former case spectroscopes must be attached to
telescopes, and hence again they must be light and small, unless a
siderostat is employed.
In the latter case the prism is placed outside the object-glass, and the
true telescope becomes the observing telescope.
Fraunhofer, at the beginning of the present century, was the first to
observe the spectra of the stars by placing a large prism outside the
object-glass, three or four inches in diameter, of his telescope, and so
virtually making the star itself the slit of the spectroscope; and in
fact he almost anticipated the arrangement of Mr. Simms, and satisfied
the conditions of the problem. The parallel light from the star passed
through the prism, and by means of the object-glass was brought to a
focus in front of the eyepiece, so that the spectrum of the star was
seen in the place of the star itself.
This system has recently been re-invented, and the accompanying woodcut,
Fig. 191, shows a prism arranged to be placed in front of an
object-glass of four inches aperture. It is seen that the angle of the
prism is very small. The objection to this method of procedure is that
the telescope has to be pointed away from the object at an angle
depending upon the angle of the prism.
[Illustration:
FIG. 191.—Object-glass Prism.
]
In the other arrangement we have the thing managed in a different way:
we have the object-glass collecting the light from the star and bringing
it to a focus on the slit, and it then passes on to the prisms, through
which the light has to pass before it comes to the eye. In this
combination of telescope and spectroscope we have what has been called
the _telespectroscope_; one method of combination is seen in the
accompanying drawing of the spectroscope attached to Mr. Newall’s great
refractor; but any method will do which unites rigidity with lightness
and allows the whole instrument to be rotated with smoothness.
[Illustration:
FIG. 192.—The Eyepiece End of the Newall Refractor (of 25 inches
aperture), with Spectroscope attached.
]
For solar observation, as there is light enough to admit of great
dispersion, many prisms are employed, as shown in Fig. 192; or the
prisms may be made so tall that the light may be sent backwards and
forwards many times by means of return prisms, to which reference has
been already made.
For the observation of those bodies which give a small amount of light,
fewer prisms must be used, and arrangements are made for the employment
of reference spectra, _i.e._, to throw the light coming from different
chemical elements into the spectroscope, in order that we may test the
lines; whether any line of Sirius, for instance, is due to the vapour of
magnesium, as Kirchhoff tested whether any line in the sunlight was
referable to iron or the other vapours which he subsequently studied.
[Illustration:
FIG. 193.—Solar Spectroscope (Browning’s form).
]
[Illustration:
FIG. 194.—Solar Spectroscope (Grubb’s form).
]
[Illustration:
FIG. 195.—Side view of Spectroscope, showing the arrangement by which
the light from a spark is thrown into the instrument by means of the
reflecting prism, _e_, by a mirror F. (Huggins.)
]
[Illustration:
FIG. 196.—Plan of Spectroscope. T, eyepiece end of telescope, B
interior tube, carrying A, cylindrical lens; D, slit of
spectroscope; G, collimating lens; _h h_, prisms; Q, micrometer.
(Huggins.)
]
[Illustration:
FIG. 197.—Cambridge Star Spectroscope Elevation.
]
[Illustration:
FIG. 198.—Cambridge Spectroscope Plan.
]
These are shown in Fig. 195. _e_ is a reflecting prism, and F is another
movable reflector to reflect the light from a spark passed between two
wires of the metal to be compared, and to throw it on the prism, which
reflects the light through the slit of the spectroscope to the prisms
and eye; if the instrument were in perfect adjustment and turned on a
star, and a person were to place his eye to the spectroscope, he would
see in one-half of the field of view the spectrum of the star with dark
lines, and in the other half the spectrum of the vapour with its bright
lines; and if he found the bright lines of the vapour to correspond with
any particular dark line of the spectrum of the star, he would know
whether the metal exists at that star or not; so this little mechanical
arrangement at once tells him what there is at the star, whether it be
iron or anything else.
In Figs. 197 and 198 is shown another form of stellar spectroscope, that
of the Cambridge (U.S.) observatory; it is the same in principle as that
just described.
A direct vision star spectroscope is shown in Fig. 199.
[Illustration:
FIG. 199.—Direct-vision Star Spectroscope. (Secchi.)
]
A new optical contrivance altogether has to be used when star spectra
are observed.
The image of a star is a point, and if focussed on the slit will of
course give only an extremely narrow spectrum; to obviate this a
cylindrical lens is employed, which may be placed either before the slit
or between the eyepiece and the eye. If placed before the slit, it draws
out the image of the star to a fine line which just fits the slit, so
that a sufficient portion of the slit is illuminated to give a spectrum
wide enough to show the lines, or the slit may be dispensed with
altogether.
In stellar observations, when the cylindrical lens is used in front of
the slit, special precautions should be taken so as to secure that the
position of the cylindrical lens and slit in which the spectrum appears
brightest should be used. In any but the largest telescopes the spectra
of the stars are so dim that unless great care is used the finer lines
will be missed. A slit is not at all necessary for merely seeing the
spectra; indeed they are best seen without one. If a slit be used, it
should lie in a parallel and not in a meridian; under these
circumstances slight variations in the rate of the clock are of no
moment.
In this and in other observational matters it is good to know what to
look for, and there are great generic differences between the spectra of
the various stars. In Fig. 200 are represented spectra from the
observations of Father Secchi. In the spectrum of Sirius, a
representative of Type I., very few lines are represented, but the lines
are very thick; and stars of this class are the easiest to observe.
Next we have the solar spectrum, which is a representative of Type II.,
one in which more lines are represented. In Type III. fluted spaces
begin to appear; and in Type IV., which is that of the red stars,
nothing but fluted spaces is visible, and this spectrum shows that there
is something different at work in the atmosphere of those red stars to
what there is in the simpler atmosphere of the first—of Type I. These
observations were first attempted, and carried on with some success, by
Fraunhofer, and we know with what skill and perseverance Mr. Huggins has
continued the work in later years, even employing reference spectra and
determining their chemical constitution as well as their class.
[Illustration:
FIG. 200.—Types of Stellar Spectra (Secchi).
]
We need scarcely say that the same arrangement, minus the cylindrical
lens, is good for observing the nebulæ and such other celestial objects
as comets and planets.
For all spectrum work, it has to be borne in mind that the best
definition is to be had when the actual colour under examination is
focussed on the slit. With reflectors, of course, there is no difference
of focus for the different colours. As the best object-glasses are
over-corrected for chromatic aberration, the red focus is generally
inside and the blue one outside the visual one. It is not necessary to
move the whole spectroscope to secure this; all collimators should be
provided with a rack and pinion giving them a bodily movement backwards
and forwards.
This precaution is of especial importance in the case of solar
observations, to which we have next to refer.
If in any portion of the sun’s image on the plate carrying the slit we
see a spot, all we have to do is to move the telescope, and with it of
course the sun’s image, so that the slit is immersed in the image of the
spot; if, however, we wish to observe a bright portion of the sun, we
can immerse this slit in the bright portion. Again, if we wish to
examine the chromosphere of the sun, we simply have to cover half the
slit with the sun, and allow the other part of the slit to be covered by
any surroundings of the sun, and, so to speak, to fish round the edge;
the lower half of the slit, say, is covered by the sun itself, and
therefore we shall get from that half the ordinary solar spectrum; the
upper half is, however, immersed in the light reflected from our
atmosphere, giving a weak solar spectrum, so that we get a bright and
feeble spectrum side by side. But besides the atmospheric light falling
on the upper part of the slit, the image of anything surrounding the sun
falls there also, and its spectrum is seen with the faint solar
spectrum, and we find there a spectrum of several bright lines. Now, as
an increase of dispersive power will spread out a continuous spectrum
and weaken it, we may almost indefinitely weaken the atmospheric
spectrum, and so practically get rid of it, still leaving the
bright-line spectrum with the lines still further separated; so that if
it were not for our atmosphere, we should get only the spectrum of the
sun and that of its surroundings; one a continuous spectrum with black
lines, and the other consisting of bright lines only.
Now if we suppose these observations made—if the precaution to which we
have alluded be not taken, the spectrum of the sun-spot will differ but
little from that of the general surface, and the chromospheric lines
will scarcely be visible.
If the precaution _be_ taken, in the case of the spot it will be found
that every one of the surrounding pores is also a spot; and if the air
be pure the spectrum will be full of hard lines running along the
spectrum, just like dust lines, but emphatically not dust lines, because
they change with every movement of the sun. The figure of the spot
spectrum on p. 415 will show what is meant. Fig. 201 will show the
appearance of the chromospheric line when the blue-green light is
exactly focussed; the boundary of the spectrum of the photosphere
approaches in hardness that at the end of the slit.
By measuring the lengths of the lines we can estimate the height of the
vapours producing them; we find from this that magnesium is usually
present to a height of a few hundred miles, and that hydrogen extends to
between 3,000 and 4,000 miles; in some positions of the slit the
hydrogen lines are seen to start up to great heights, showing the
presence of flames or prominences extending in height to sometimes
100,000 miles.
[Illustration:
FIG. 201.—Part of Solar Spectrum near F.
]
If, without changing the focus, we open the slit wider, and throw the
sun’s image just off the slit, so that the very bright continuous
spectrum no longer dazzles the eye, we shall be able to see these flames
whenever they cross the opening, for the image of the slit is focussed
on the eye, and the sun and its flames are focussed on the slit, so if
we virtually remove the slit by opening it wide, we see the flames;
still the limit of opening is soon approached, and the flood of
atmospheric light soon masks them. The red hydrogen line of the spectrum
is the best for viewing them, although the yellow or blue will answer.
We may also place the sun’s image so that the slit is tangential to it,
in which case a greater length of the hydrogen layer, or chromosphere,
as it is called, is visible, although its height is limited by the
opening of the slit.
By these means we are able to view a small part of the chromosphere at a
time, and to go all round the sun in order to obtain a daily record of
what is going on. If, however, we throw the image of the sun on a disc
of metal of exactly the same size, we eclipse the sun, but allow the
light of the chromosphere to pass the edge of the disc; this of course
is masked by the atmospheric light, but if the annulus, or ring of
chromosphere, be reduced sufficiently small, it can be viewed with a
spectroscope in the place of a slit, in fact it is virtually a circular
slit on which the chromosphere rests. By this means nearly the whole of
the chromosphere can be seen at once. This is accomplished as follows:—
The image of the sun is brought to focus on a diaphragm having a
circular disk of brass in the centre, of the same size as the sun’s
image, so that the sun’s light is obstructed and the chromospheric light
is allowed to pass. The chromosphere is afterwards brought to a focus
again at the position usually occupied by the slit of the spectroscope;
and in the eyepiece is seen the chromosphere in circles corresponding to
the “C” or other lines.
A lens is used to reduce the size of the sun’s image, and keep it of the
same size as the diaphragm at different times of the year; and other
lenses are used in order to reduce the size of the annulus of light to
about ⅛ inch, so that the pencils of light from either side of it may
not be too divergent to pass through the prisms at the same time, in
order that the image of the whole annulus may be seen at once. There are
mechanical difficulties in producing a perfect annulus of the required
size, so one ½ inch in diameter is used, and can be reduced virtually to
any size at pleasure.
From what has been said it is easy to see that we really now get a new
language of light altogether, and a language which requires a good deal
of interpretation.
[Illustration:
FIG. 202.—Distortions of F line on Sun.
]
We have still, indeed, to consider some curious observations which are
now capable of being made every day when anything like a sun-storm is
going on, by means of the arrangement in which the spectroscope simply
deals with the light that comes from a small portion of the sun instead
of from all the sun. If we make the slit travel over different portions
of the sun on which any up-rushes of heated material, or down-rushes of
cold material, or other changes, are going on from change of surface
temperature, the Fraunhofer lines, which we have before shown to be
straight, instead of being so, appear contorted and twisted in all
directions. On the other hand, if we examine the chromosphere under the
same conditions, we find the bright lines contorted in the same manner.
The usually dark lines, moreover, sometimes appear bright, even on the
sun itself; sometimes they are much changed in their relative positions
with reference to the solar spectrum. The meaning of these contortions
has already been hinted at (p. 420).
It was there shown that every colour, or light of every refrangibility,
is placed by the prisms in its own particular position, so if a ray of
light alters its position in the spectrum it must change its colour or
refrangibility, so the light producing the F line in the one case, and
the absent light producing the dark line in the other, differ slightly
in colour, or are rather more or less refrangible than the normal light
from hydrogen. In the case when the F line is wafted towards the blue
end of the spectrum, the light falling on the slit is rather more
refrangible than usual; and in the middle drawing, Fig. 203, where the F
line bifurcates, the slit is supplied with two kinds of light differing
slightly in refrangibility. Not only does the light radiated by a
substance change in this way, but the light absorbed by that substance
also changes, hence the contortions of the black lines are due to a
similar cause.
[Illustration:
FIG. 203.—Displacement of F line on edge of Sun.
]
Here, therefore, we have evidence of a change of refrangibility, or
colour, of the light coming from the hydrogen surrounding the sun. This
change of refrangibility is due to the motion of the solar gases, as
explained in the last chapter.
So we find that the hydrogen producing the light giving us one of the
forms of the F line, shown in Fig. 203, is moving towards us at the rate
of 120 miles a second, while that giving the other form is moving away
from us.
Let us see how these immense velocities are estimated. By means of
careful measurements, Ångström has shown on his map of the solar
spectrum the absolute length of the waves of light corresponding to the
lines; thus the length of the wave of light of hydrogen giving the F
line is 4860/10000000 of a millimeter. In Fig. 203 the dots on either
side of the F line show the positions, where light would fall, if it
differed from the F light by 1, 2, 3, or 4 ten-millionths of a
millimeter, so that in the figure the light of that part of the line
wafted over the fourth dot is of a wave-length of 4 ten-millionths of a
millimeter less than that of the normal F light, which has a wave-length
4860/10000000 of a millimeter. The F light therefore has had its
wave-length reduced by 4/4860 = 1/1215 part; and in order that each wave
may be decreased by this amount, the source of the light must move
towards us with a velocity of 1/1215 of the velocity of light, which is
186,000 miles per second, and 1/1215 of 186,000 is about 150; this then
is the velocity, in miles per second, at which the hydrogen gas must
have been moving towards us in order to displace the light to the fourth
dot, as shown in the figure.