to invite the reader’s attention. The power of certain gases to absorb
or stop certain rays of an otherwise continuous spectrum has already
been mentioned; but this property is by no means confined to gases, for
certain liquids and solids do this in a high degree. There is a
remarkable metallic element, named _didymium_. It is a rare substance,
and its presence cannot with certainty be detected by any ordinary
tests. Its salts, however, form solutions _without colour_, or nearly
so, which have the power of strongly absorbing certain rays. If we hold
before the slit of the spectrum a small tube containing a solution of
any one of the salts, and allow the rays from the sun, or from a
luminous gas or candle-flame, to pass through it, we see the spectrum
crossed by certain well-defined very dark bands. A spectrum of this kind
is called an _absorption spectrum_, and the position, number, width,
&c., of dark bands are found to be as peculiar to each substance as are
the bright lines in the spectra of the elements. The method of observing
them when produced by solutions is very simple. The liquid is contained
in a small test-tube, which is placed in front of the slit; or, more
conveniently, the liquid is put into a _wedge-shaped_ vessel, and thus
the thickness of the stratum of liquid through which the rays pass can
easily be varied, so that the best results may be obtained. The
absorption spectra are produced by many compound substances. A striking
absorption spectrum is seen when a solution in alcohol of the green
colouring matter of leaves (_chlorophyll_) is examined; for several
distinct bands are seen, one in the red being especially well marked.
Many other coloured bodies exhibit characteristic absorption bands, as,
for example, permanganate of potash, uranic salts, madder, port wine,
and magenta. The bands are so peculiar for each substance, that if
so-called port wine, for example, owe its colour to colouring matter
other than that of the grape, such as logwood, &c., the adulteration can
be instantly detected by a glance at the absorption spectrum. As,
however, the absorption bands are not, like the bright lines of metals,
definite images of the slit, but rather broad portions of the spectra,
it is very desirable in examining such spectra to compare them directly
with those of known substances, by throwing two spectra into one field,
by means of a side reflecting prism, as already described.
Perhaps one of the most interesting examples of absorption spectra is
that of blood. A single drop of blood in a tea-cupful of water will show
its characteristic spectrum when it is properly examined. If the blood
is arterial or oxidized blood, two well-marked dark bands are visible;
but if venous or deoxidized blood be used, we see, instead of the two
dark bands, a single one in an intermediate position. These differences
have been proved to be due to oxidization and deoxidization of a
constituent of the blood, called _hæmoglobin_, and by using appropriate
chemical reagents, the same specimen of blood may be made to exhibit any
number of alternations of the two spectra, according as oxidants or
reducing reagents are employed. It would be possible by an examination
of the absorption spectrum of a drop of arterial blood to pronounce that
a person had died of suffocation from the fumes of burning charcoal. In
such case, the supply of oxygen being cut off, the hæmoglobin of the
whole of the blood in the system becomes deoxidized.
The beautiful delicacy of these spectrum reactions has permitted the
spectroscope to be applied to the microscope with signal success by Mr.
Browning, working in conjunction with Mr. Sorby, who has devoted great
attention to this subject. The Sorby-Browning instrument is a
direct-vision spectroscope, with a slit, lens, &c., placed above the
eye-piece of the microscope. By receiving the light through a single
drop of an absorptive liquid placed under the object-glass of the
microscope, the characteristic bands are made visible. The
micro-spectroscope is also a valuable instrument for examining the
absorption bands which are found in the light reflected from solid
bodies, for the smallest fragment suffices to fill the field of the
microscope. Mr. Sorby is able to obtain most unmistakably the dark bands
peculiar to blood from a particle of the matter of a blood-stain
weighing less than 1/1000th part of a grain. It is plain from this that
the spectroscope must sometimes prove of great service in giving
evidence of crime from traces which would escape all ordinary
observation.
The micro-spectroscope, in its most complete form, is represented in
Fig. 224. As may be seen from the figure, the apparatus consists of
several parts. The prism is contained in a small tube, which can be
removed at pleasure; below the prism is an achromatic eye-piece, having
an adjustable slit between the two lenses; the upper lens being
furnished with a screw motion to focus the slit. A side slit, capable of
adjustment, admits, when required, a second beam of light from any
object whose spectrum it is desired to compare with that of the object
placed on the stage of the microscope. This second beam of light strikes
against a very small prism suitably placed inside the apparatus, and is
reflected up through the compound prism, forming a spectrum in the same
field with that obtained from the object on the stage. A is a brass tube
carrying the compound direct-vision prism, and has a sliding arrangement
for roughly focussing.
[Illustration:
FIG. 224.—_The Sorby-Browning Micro-Spectroscope._
]
B, a milled head, with screw motion to finely adjust the focus of the
achromatic eye-lens.
C, milled head, with screw motion to open or shut the slit _vertically_.
Another screw, H, at right angles to C, regulates the slit horizontally.
This screw has a larger head, and when once recognized cannot be
mistaken for the other.
D D, an apparatus for holding a small tube, that the spectrum given by
its contents may be compared with that from any other object on the
stage.
E, a screw, opening and shutting a slit to admit the quantity of light
required to form the second spectrum. Light entering the aperture near E
strikes against the right-angled prism which we have mentioned as being
placed inside the apparatus, and is reflected up through the slit
belonging to the compound prism. If any incandescent object is placed in
a suitable position with reference to the aperture, its spectrum will be
obtained, and will be seen on looking through it.
F shows the position of the field lens of the eye-piece.
G is a tube made to fit the microscope to which the instrument is
applied. To use this instrument, insert G like an eye-piece in the
microscope tube. Screw on to the microscope the object-glass required,
and place the object whose spectrum is to be viewed on the stage.
Illuminate with stage mirror if transparent, with mirror and lieberkühn
and dark well if opaque, or by side reflector, bull’s-eye, &c. Remove A,
and open the slit by means of the milled head, H, at right angles to D
D. When the slit is sufficiently open the rest of the apparatus acts
like an ordinary eye-piece, and any object can be focussed in the usual
way. Having focussed the object, replace A, and gradually close the slit
till a good spectrum is obtained. The spectrum will be much improved by
throwing the object a little out of focus.
Every part of the spectrum differs a little from adjacent parts in
refrangibility, and delicate bands or lines can only be brought out by
accurately focussing their own parts of the spectrum. This can be done
by the milled head, B. Disappointment will occur in any attempt at
delicate investigation if this direction is not _carefully attended to_.
When the spectra of very small objects are to be viewed, powers of from
½ in. to 1/20th, or higher, may be employed. Blood, madder, aniline
dyes, permanganate of potash solution, are convenient substances to
begin experiments with. Solutions that are too strong are apt to give
dark clouds instead of delicate absorption bands. Small cells or tubes
should be used to hold fluids for examination.
Mr. Browning has still further improved the micro-spectroscope by the
ingenious arrangement for measuring the positions of the lines, which is
represented in Fig. 225, and the construction and the use of which he
thus described in a paper read before the Microscopical Society:
[Illustration:
FIG. 225.—_Section of Micro-Spectroscope with Micrometer._
]
Attached to the side is a small tube, A A. At the outer part of this
tube is a blackened glass plate, with a fine clear white pointer in the
centre of the tube. The lens, C, which is focussed by sliding the milled
ring, M, produces an image of the bright pointer in the field of view by
reflection from the surface of the prism nearest the eye. On turning the
micrometer, M, the slide which holds the glass plate is made to travel
in grooves, and the fine pointer is made to traverse the whole length of
the spectrum.
It might at first sight appear as if any ordinary spider’s web or
parallel wire micrometer might be used instead of this contrivance. But
on closer attention it will be seen that as the spectrum will not permit
of magnification by the use of lenses, the line of such an ordinary
micrometer could not be brought to focus and rendered visible. The
bright pointer of the new arrangement possesses this great
advantage—that it does not illuminate the whole field of view.
If a dark wire were used, the bright diffused light would almost obscure
the faint light of the spectra, and entirely prevent the possibility of
seeing, let alone measuring, the position of lines or bands in the most
refrangible part of the spectrum.
To produce good effects with this apparatus the upper surface of the
compound prism, P, must make an angle of exactly 45° with the sides of
the tube. Under these circumstances the limits of correction for the
path of the rays in their passage through the dispersing prisms are very
limited and must be strictly observed. The usual method of correcting by
the outer surface is inadmissible. For the sake of simplicity, some of
the work of the lower part of the micro-spectroscope is omitted in the
engraving. As to the method of using this contrivance: With the
apparatus just described, measure the position of the principal
Fraunhofer’s lines in the solar spectrum. Let this be done _carefully_,
in _bright_ daylight. A little time given to this measurement will not
be thrown away, as it will not require to be done again. Note down the
numbers corresponding to the position of the lines, and draw a spectrum
from a scale of equal parts. About 3 in. will be found long enough for
this spectrum; but it may be made as much longer as is thought
desirable, as the measurements will not depend in any way on the
distance of these lines apart, but only on the micrometric numbers
attached to them. Let this scale be done on cardboard and preserved for
reference. Now measure the position of the dark bands in any absorption
spectra, taking care for this purpose to use lamplight, as daylight will
give, of course, the Fraunhofer lines, which will tend to confuse your
spectrum. If the few lines occurring in most absorption spectra be now
drawn to the same scale as the solar spectrum, on placing the scales
side by side, a glance will show the exact position of the bands in the
spectrum relatively to the Fraunhofer lines, which thus treated form a
natural and unchangeable scale (see diagram, Fig. 226). But for purposes
of comparison it will be found sufficient to compare the two lists of
numbers representing the micrometric measures, simply exchanging copies
of the scale of Fraunhofer lines, or the numbers representing them will
enable observers at a distance from each other to compare their results,
or even to work simultaneously on the same subject.
[Illustration:
FIG. 226.
]
A simpler form of the micro-spectroscope is also made by Mr. Browning at
a very modest price, and if the reader possesses a microscope, and
desires to examine these interesting subjects for himself, he will do
well to procure this instrument, instead of that represented in Fig.
220, as it will also answer better for other purposes. A section of the
instrument is shown in Fig. 227. When used with the microscope it is
slipped into the place of the eye-piece. There is an adjustable slit, a
reflecting prism, by which two different spectra may be examined at
once, and a train of five prisms for dispersing the rays. It can be used
equally well for seeing the bright lines of metals and the Fraunhofer
lines, and for viewing any two spectra simultaneously. These
direct-vision spectroscopes are better adapted for general use by those
who have not several different instruments, than such forms as that
shown in Fig. 229, for in the direct-vision instruments the whole extent
of the spectrum is visible at one view, which is by no means the case
with the larger instruments.
[Illustration:
FIG. 227.—_Section of Micro-Spectroscope._
]
_CELESTIAL CHEMISTRY AND PHYSICS._
We now approach that portion of our subject in which its interest
culminates, for however remarkable may be some of the above-named
results of this searching optical analysis, they are surpassed by those
which have been obtained in the field upon which we are about to enter.
The cause of the dark lines which Fraunhofer observed in the light of
the sun and of certain stars remained unexplained, he only establishing
the fact that they must be due to some absorptive power existing in the
sun and stars themselves, and not to anything in our atmosphere. It was
reserved for Professor Kirchhoff, of the University of Heidelberg, to
show the full significance of the dark lines. Fraunhofer had, on his
first observation of the lines, noticed that the D lines were coincident
with the bright lines in the spectrum of sodium. This interesting fact
may be readily observed with any spectroscope which permits of the two
spectra being simultaneously viewed. The bright line (or lines if the
spectroscope be powerful) of the metal is seen as a prolongation of the
dark D solar line. Even with an instrument like that shown in Fig. 220
the coincidence may be noticed. Let the observer receive into the
instrument the rays in diffused daylight only, when he will still see
the principal Fraunhofer lines distinctly, and let him note the exact
position of the D line, while he brings in front of the slit the flame
of a spirit-lamp charged with a little salt. He will then see the bright
yellow line replacing the dark D line, and by alternately removing and
putting back the lamp he will be soon convinced of the perfectly
identical position of the lines.
This fact remained without explanation from 1814 to 1859, when Kirchhoff
accidentally found, to his surprise, that the dark D line could be
produced artificially. He says: “In order to test in the most direct
manner possible the frequently asserted fact of the coincidence of the
sodium lines with the D lines, I obtained a tolerably bright solar
spectrum, and brought a flame coloured by sodium vapour in front of the
slit. I then saw the dark lines D, change into bright ones. The flame of
a Bunsen’s lamp threw the bright sodium lines upon the solar spectrum
with unexpected brilliancy. In order to find out the extent to which the
intensity of the solar spectrum could be increased without impairing the
distinctness of the sodium lines, I allowed the full sunlight to shine
through the sodium flame, and, to my astonishment, I saw that the _dark
lines_, D, _appeared with an extraordinary_ degree of clearness. I then
exchanged the sunlight for the Drummond’s or oxy-hydrogen lime-light,
which, like that of all incandescent solid or liquid bodies, gives a
spectrum containing no dark lines. When this light was allowed to fall
through a suitable flame, coloured by common salt, _dark_ lines were
seen in the spectrum in the position of the sodium lines. The same
phenomenon was observed if, instead of the incandescent lime, a platinum
wire was used, which, being heated in the flame, was brought to a
temperature near its melting point, by passing an electric current
through it. The phenomenon in question is easily explained, upon the
supposition that the sodium flame absorbs rays of the same degree of
refrangibility as those it emits, whilst it is perfectly transparent for
all other rays.” (Quoted in Roscoe’s Lectures on “Spectrum Analysis.”)
When the light of ignited lime was similarly made to pass through flames
containing the incandescent vapours of potassium, barium, strontium,
&c., the bright lines which these substances would have produced had the
lime-light not been present were found to be in every case changed into
dark lines, occupying the very same positions in the spectrum. In such
experiments the flames containing the metals in the vapourized state do
all the time really give off those rays which are peculiar to each
substance; but when a more intense illumination—such as the lime-light,
the electric arc, or direct sunlight—passes through them, the rays of
the spectrum produced by the intense light overpower those given off by
the relatively feebly coloured flames, and hence the portions of the
spectrum which are occupied by these, appear black. But as the intense
light would give a perfectly continuous spectrum if the incandescent
metallic vapour allowed the rays corresponding to its lines to pass
through it, the inference is obvious that each vapour absorbs those
particular rays which it has itself the power of emitting, but allows
all others to pass freely through it. Besides the experimental proofs of
this fact which have been already adduced, many others might be named.
The flame of a spirit-lamp with a salted wick appears opaque and smoky
when we look through it at a large flame of burning hydrogen, also
coloured by sodium; for the rays emitted by the latter do not penetrate
the former, which, in consequence of its feebler light, appears dark by
comparison. Again, if an exhausted tube containing metallic sodium be
heated so as to convert the sodium into vapour, the tube viewed by the
light of a sodium flame appear to contain a black smoke, and the light
from the flame will no more pass through it than through a solid object;
yet the tube appears perfectly transparent when viewed by ordinary
light, and the light from a lithium or other coloured flame would also
pass freely. Kirchhoff was led by purely theoretical reasoning to
conclude that all luminous bodies have precisely the same power of
absorbing certain rays of light as they have of emitting them at the
same temperature, and he thus brought luminous rays under the same
general law which had previously been established for radiant heat by
Prevost, Dessains, Balfour Stewart, and others. Here, then, a law was
arrived at, and, abundantly confirmed by direct experiment as regards
the more volatile metals, it was ready to supply the most satisfactory
explanation of the coincidences which were everywhere discovered to
exist between the Fraunhofer lines and those which belong to terrestrial
substances. For Kirchhoff also found, when mapping the very numerous
lines seen in the spark spectrum of iron, that for each of the 90 bright
lines of iron which he then observed, there was a dark line in the solar
spectrum exactly corresponding in position. The number of observed
bright lines in the iron spectrum has been since extended to 460, and
yet each is found to have its exact counterpart in a solar dark line.
So many coincidences as these made it certain that these dark lines and
the bright lines of iron must have a common cause, for the chances
against the supposition that the agreement was merely accidental are
enormous. Kirchhoff actually calculated, by the theory of probabilities,
the odds against the supposition. He found it represented by
1,000,000,000,000,000,000 to 1. The result arrived at in the case of
sodium at once suggested the explanation that these lines were produced
by an absorptive effect of the vapour of iron. Now, the existence of
such a vapour in our atmosphere could not be admitted, while the
temperature of the sun was known to be exceedingly high, far higher,
indeed, than any temperature we can produce by electricity, or any other
means. Hence, Kirchhoff concluded that his observations proved the
presence of the vapour of iron in the sun’s atmosphere with as much
certainty as if the iron had been actually submitted to chemical tests.
By the same reasoning, Kirchhoff also demonstrated the existence in the
solar atmosphere of calcium, chromium, magnesium, nickel, barium,
copper, and zinc. To these, other observers have added strontium,
cadmium, cobalt, manganese, lead, potassium, aluminium, titanium,
uranium, and hydrogen. It has also been demonstrated that a considerable
number of the Fraunhofer lines are due to absorption in our atmosphere
by its gases and aqueous vapour. This demonstration of the existence of
iron and nickel in the sun is an interesting pendent to the known
composition of many meteorites which reach us from interplanetary space.
Kirchhoff was led to believe that the central part of the sun is formed
of an incandescent solid or liquid, giving out rays of all
refrangibility, just as white-hot carbon does; that round this there is
an immense atmosphere, in which sodium, iron, aluminium, &c., exist in
the state of gas, where they have the power of absorbing certain rays;
that the solar atmosphere extends far beyond the sun, and forms the
corona; and that the dark sunspots, which astronomers have supposed to
be cavities, are a kind of cloud, floating in the vaporous atmosphere.
During total eclipses of the sun, certain red-coloured prominences have
been noticed projecting from the sun’s limb, and visible only when the
glare of its disc is entirely intercepted by the moon. Fig. 228
represents a total eclipse, and will give a rude notion of the
appearance of the red prominences seen against the fainter light of the
_corona_, which extends to a considerable distance beyond the sun’s
disc. Now, two distinguished men of science simultaneously and
independently made the discovery of a mode of seeing these red
prominences, even when the sun was unobscured. M. Janssen was observing
a total eclipse of the sun in India, and the examination by the
spectroscope of the light emitted from the red prominences showed him
that they were due to immense columns of incandescent hydrogen, for he
recognised the red line and blue lines which belong to the spectrum of
this gas (see No. 12, Plate XVII.). Mr. Norman Lockyer at the same time
also succeeded in viewing the solar prominences in London without an
eclipse. He found a red line perfectly coinciding in position with
Fraunhofer’s C line and that of hydrogen, another nearly coinciding with
F, and a third yellow line near D. Soon after this, Dr. Huggins
discovered a mode of observing the shape of the red prominences at any
time, by using a powerful train of prisms and a wide slit, so that the
changes in the forms of the red flames can be followed. Now, since the
red prominences give off only a few rays of particular refrangibility,
it is not difficult to understand that the light of the sun might be, as
it were, so diluted by stretching out the spectrum, by means of a train
of many prisms, that almost only the red rays, C, should enter the
telescope, and occupy the field with sufficient intensity to overpower
all others, and produce an image of the object from which they
originated. The nature of this action may be illustrated thus: If we
hold vertically a prism, and look through it at a candle-flame, we may
perceive a lengthened-out image of the flame, showing the succession of
prismatic colours, and formed, as it were, of a red image of the flame
close to a yellow one, and so on, but presenting no defined form. If,
still viewing this spectrum, we introduce into the flame on a platinum
wire a piece of common salt, we shall perceive a well-defined yellow
image of the candle start out, because the rays which are emitted by the
incandescent sodium, being all of one refrangibility, the prism simply
refracts without dispersing them. The dispersion which weakens the light
of the continuous spectrum by lengthening it out, does not sensibly
detract from the brilliancy of the bright lines, as their breadth is
scarcely increased—they are refracted but not dispersed. Hence, when a
sufficient number of prisms is employed, the bright lines of the solar
_chromosphere_ may be seen in full sunshine, in spite of the greater
intensity of the light emanating from the _photosphere_, which produces
the continuous spectrum. The bright C line is, of course, a virtual
image of the slit produced by rays of that particular refrangibility;
but by using a very high dispersive power, the slit may be opened so
wide that the C rays form in the telescope a red image of the prominence
from which they issue, since their light will predominate over that of
any rays belonging to the continuous spectrum.
[Illustration:
FIG. 228.—_Solar Eclipse, 1869._
]