THE CHEMISTRY OF THE STARS: CONSTRUCTION OF THE SPECTROSCOPE.
In the addition of chemical ideas to astronomical inquiries, we have one
of the most fruitful and interesting among the many advances of modern
science, and one also which has made the connection between physics and
astronomy one of the closest.
To deal properly with this part of our book, as the constitution of one
of the heavenly bodies can be studied in the laboratory as well as in
the observatory, we have to describe physical instruments and methods,
as well as the more purely astronomical ones.
In a now rare book published in London in the year 1653, that is to say,
some years before Sir Isaac Newton made his important observations on
the action of a prism on the rays of light—observations which have been
so very rich in results—is given Kepler’s treatise on Dioptrics. From
this one finds that the great Kepler had done all he could to try to
investigate the action of a three-cornered piece of glass.
It has been considered, that, because Newton was the first to teach us
much of its use, he was the first to investigate the properties of the
prism. This is not so. Fig. 167 is an illustration taken from this book,
by which Kepler shows that if we have a prism and pass light through it,
we get three distinct results when a ray (F) falls on the prism. He
shows that the first surface reflects a certain amount of light, (D I),
and that this is uncoloured, because it does not pass through the glass,
and that the remainder is refracted by the glass and part emerges at E,
coloured like the rainbow. Then he goes on to show that the second
surface of the prism also reflects some light internally, and that there
is a certain amount of light leaving the prism at M, and going to K.
[Illustration:
FIG. 167.—Kepler’s Diagram.
]
By means of a very few experiments Newton was able to show how much
knowledge could be got by examination of the prism. The first
proposition in Newton’s _Optics_ is an attempt to prove that light,
which differs in colour, differs also in degree of refrangibility. We
shall recollect from the fifth chapter what this term means, for it was
there shown that whenever a ray of light enters obliquely a medium
denser than that in which it had been travelling, it is bent towards the
perpendicular to the surface, in fact it is refracted, and those rays
which are most refracted by the same substance with the same angle are
said to be more refrangible than others. Newton’s experiment was very
simple. He took a piece of paper, one half of which was coloured red and
the other half blue; and this was placed on a stand horizontally, in the
light from a window, with a prism between it and the eye.
[Illustration:
FIG. 168.—Newton’s Experiment showing the different Refrangibilities
of Colours.
]
He went on to show, that when he allowed the beam of sunlight to fall
upon the paper, strongly illuminating the red and blue portions, making
at the same time all the rest of the room as dark as possible (so that
the operation was not impeded by extraneous light), when he held a prism
in a particular way, he found that the red and the blue occupied
different positions when looked at through the prism. When the prism is
held as shown, the red is seen below and the blue above. If the prism be
turned with the refracting edge downwards, the red is seen above and the
blue below. When the refracting edge is upwards, it is very clear that
if the violet is seen uppermost it must be because the violet ray is
more refracted, and when the red ray is uppermost, with the refracting
edge of the prism downwards, it is because the red ray is the least
refracted.
There are other experiments to which he alludes, and by which Sir Isaac
Newton considered he had proved that lights which differ in colour
differ also in degrees of refrangibility.
Newton at one step went to the sun, and his second theorem is “The light
of the sun consists of rays of different refrangibility,” and then he
enters into the proof by experiment. The light from the sun passes
through a hole in the window-shutter and through the prism which throws
a spectrum on a screen. We now see the full meaning of the different
degrees of refrangibility. There he had a long band of light of all
colours, the red at one end and the blue at the other, showing that the
different colours are unequally refracted, or turned from their course.
In this way Sir Isaac Newton determined whether the law, that light
which differed in colour differed also in refrangibility, held true with
regard to the sun; and he clearly showed that in this case also the
light differs in refrangibility, in exactly the same way as the red
light and the blue light had done in his experiment with the pieces of
paper. He was soon able to prove to himself that the circular aperture
was not the best thing he could use, because in the spectrum he had a
circle of colour representing every ray into which the light could be
broken up. If we put a bit of red glass in the path of the rays we get
an image of the hole in red; if we use other coloured glasses, we have a
circle for each particular colour; all these images overlap, and the sum
total gives us an extremely mixed spectrum, something quite different
from what is seen when we introduce a slight alteration, which curiously
enough was delayed for a great many years.
Sir Isaac Newton recognised the difficulties there were in getting a
pure spectrum by means of a circular aperture, but although he used
afterwards an oblong opening instead of a circular aperture, in which we
had something more or less like what we now use, namely, a “slit”—a
narrow line of light; he does not seem to have grasped the point of the
thing, because in one of his theorems he says he also tried triangular
openings. We shall show how important it is that we should not only have
an oblong opening as proposed by Newton, but that that oblong opening
should be of small breadth.
The moment we exchange the circular aperture for the oblong opening of
Newton, we get a spectrum of greater purity, and, as in the case of the
circular opening the purity depended on the size of the circle, so also
in the case of the oblong opening the purity of the spectrum depends
very much on the breadth of the oblong opening.
We thus sort out the red, orange, yellow, green, blue, and violet; they
are no longer mixed as they are when we employ a circular opening. If we
attempt the same experiment with red glass interposed we get something
more decided than before; we have no longer a circular patch of light,
but an oblong one in the red; in fact, the exact form of the aperture,
or slit, through which we have allowed the light to pass through the
prism and lens to form an image.
[Illustration:
FIG. 169.—Wollaston’s first Observation of the Lines in the Solar
Spectrum.
]
Now although Newton made these important observations on sunlight, he
missed one of the things, in fact we may say _the_ thing, which has made
sunlight and starlight of so much importance to Astronomy. The oblong
opening which Newton used varied from one-tenth to one-twentieth of an
inch in width; but Dr. Wollaston in 1812—we had to wait from 1672 till
1812 to get this apparently ridiculously small extension—used such a
narrow slit as we have mentioned, and he found that when he examined the
light of the sun with a prism before the eye, he got results of which
Newton had never dreamt.
Dr. Wollaston not only found the light of the sun differing in
refrangibility; but in the different colours of the solar light he found
a number of dark lines, which are represented by the black lines across
the spectrum in Fig. 169.
[Illustration:
FIG. 170.—Copy of Fraunhofer’s first Map of the Lines in the Solar
Spectrum.
]
[Illustration:
FIG. 171.—Student’s Spectroscope.
]
In the year 1814 Fraunhofer examined the spectrum by means of the
telescope of a theodolite, directing it towards a distant slit, with a
prism interposed. In this manner he observed and mapped 576 lines, the
appearance of the spectrum to him being represented in Fig. 170. From
this time they were called the “Fraunhofer lines.” It need scarcely be
said that from the time of Wollaston until a few years ago these strange
mysterious lines were a source of wonder to all observers who attempted
to attack the problem. The difference between the simple prism and slit
which Newton, Wollaston, and Fraunhofer used to map these lines, and the
modern spectroscope, as used with or without the telescope, is due to a
suggestion of Mr. Simms in 1830.
Let us refer to a modern spectroscope. Fig. 171 represents a form
usually used for chemical analysis. The only difference between the
spectroscope and the simple prism in Newton’s experiment is this, that
in the one case the light falls directly from the slit through the prism
on a screen and is viewed there; and in the other the eye is placed
where the screen is, and looks through the prism and certain lenses at
the slit.
The great improvement which Mr. Simms suggested was this simple one. He
said, “It would surely be better that the light which passes through the
prism or prisms independently of the number I use, should, if possible,
pass through them as a parallel beam of light; and therefore, instead of
putting the slit merely on one side of a prism and the eye on the other,
I will, between the slit and the prism, insert an object-glass,” as
shown in Fig. 172; so that the slit of the spectroscope is the
representative of the hole in the shutter.
[Illustration:
FIG. 172.—Section of a Spectroscope, showing the Path of the Ray from
the Slit.
]
The slit is exactly in the focus of the little object-glass, C, or
collimating lens, as it is called; so that naturally the light is
grasped by this lens, and comes out in a parallel beam, and travels
among the prism or prisms, quite irrespective of course of their number.
This parallel beam, in order to be utilized by the eye after it has
passed through the system of prisms, is again taken up by another
object-glass and reduced from its parallel state into a state of
convergence, and brought to a focus which can be examined by means of an
eyepiece.
The red rays from the slit come to a focus at R, and the blue at B,
forming there their respective images of the slit, and between B and R
are a number of other images of the slit, painted in every colour that
is illuminating it, thus forming a spectrum which is viewed by the
eyepiece. In fact, the object-glass and eyepiece constitute a telescope,
through which the slit is viewed, and the collimating lens makes the
light parallel, just as if it had come from a distant object, and fit to
be utilized in the telescope. This is the principle to be observed in
the construction of every spectroscope.
We have now given an idea of the general nature of the instrument
depending on this important addition made by Mr. Simms, which is the
basis of the modern spectroscope, and it is obvious that if we want
considerable dispersion, we can either increase the number of prisms, or
increase their dispersive power.
We have already shown in a previous chapter that the dispersion depends
on the angle of the prisms, and that the calculations necessary for
making the object-glass of a telescope were based upon an observation
made by passing light through a prism of a particular angle made of the
same glass as that of which the proposed object-glass was to be
constructed. Then, again, we took the opportunity of showing that with
very dense substances greater dispersion could be obtained. We showed
how the prism of dense flint glass overpowered the dispersion of the
prism of the crown glass, and how the combination gave us refraction
without dispersion.
[Illustration:
FIG. 173.—Spectroscope with Four Prisms.
]
Fig. 173 is a drawing of a spectroscope containing four prisms. It is a
representation of that used by Bunsen and Kirchhoff when they made their
maps of the solar spectrum: it is so arranged that the light after
passing through the slit goes through the collimating lens, and then
through the prisms; it is afterwards caught by the telescope lens and
brought to a focus in front of the eyepiece. It is very important, when
we have many prisms, to be able to arrange them so that whether we use
one part of the spectrum or the other, each prism shall be in the best
condition for allowing the light to traverse it; that is to say, that it
shall be in the position of _minimum deviation_, when the angles of
incidence and emergence are equal, and each surface refracts the ray
equally. They can be arranged so, that as the telescope is moved to
observe a new part of the spectrum, every prism will be automatically
adjusted.
To insure this the prisms are united to form a chain so that they all
move together, and each has a radial bar to a central pin which keeps
them at the proper angle.
[Illustration:
FIG. 174.—Automatic Spectroscope (Grubb’s form).
]
There is another arrangement which is very simple, in which we get the
condition of minimum deviation by merely mounting the prisms on a
spring, and then moving the spring with the telescope, in the same way
as the telescope moves the other automatic arrangement.
[Illustration:
FIG. 175.—Automatic Spectroscope (Browning’s form).
]
For some observations, especially solar observations, in which the light
is very intense, it is extremely important, in fact essential, to reduce
the brilliancy of the spectrum; and of course this enables us, in the
case of the sun especially, to increase the dispersion almost without
limit, by having a great number of prisms, or even using the same twice
over, in the following manner:
On the spectroscope there is a number of prisms so arranged that the
light comes from the slit, and travels through the lower portion of the
prisms; it then strikes against the internal reflecting surface of a
right-angled prism at the back of the last prism, Fig. 176, and is sent,
up to another reflecting surface, and then comes back again through the
same prisms along an upper storey, and then is caught by means of a
telescope above the collimator, on the slit of which the sun’s image is
allowed to fall.
[Illustration:
FIG. 176.—Last Prism of Train for returning the Rays.
]
This contrivance, suggested by the author and Prof. Young independently,
is now largely used. Fig. 177 shows an ordinary spectroscope so armed.
The light from the slit traverses the upper portions of the prisms; it
is then thrown down by the reflecting prism seen behind the collimator,
then, returning along the lower part, it is received by a right-angled
prism in front of the object-glass of the observing telescope.
Instead of the rays of light being reflected back through the upper
storey of the prisms, another method has been adopted; the last prism is
in this case a half prism, and the last surface on which the rays of
light fall is silvered; the rays then are returned on themselves, and,
when the instrument is adjusted, come to a focus on the inside of the
slit plate, forming there a spectrum, any part of which can, by moving
the prisms, be made to fall on a small diagonal reflecting prism on one
side of the slit, by which it is reflected to the eyepiece. In this
arrangement the collimating lens becomes its own telescope lens on the
return of the ray.
[Illustration:
FIG. 177.—Spectroscope with returning Beam.
]
There is another form of spectroscope, called the _direct vision_, which
is largely used for pocket instruments. The principle of it is that the
light passing through it is dispersed but not turned from its course,
just the reverse of the achromatic combination of the object-glass; a
crown-glass prism is cemented on a flint one of sufficient angle that
their deviative powers reverse each other but leave a certain portion of
the flint-glass dispersion uncorrected; since, however, the dispersive
power of the flint-glass is to a great extent neutralized, therefore, in
order to make the instrument as powerful as one of the ordinary
construction, a number of flint-glass prisms are combined with
crown-glass ones, as shown in Fig. 178.
[Illustration:
FIG. 178.—Direct Vision Prism.
]
There is another form of direct-vision prism, called the
Herschel-Browning, in which the ray is caused to take its original
course on emerging by means of two internal reflections.