rotation, using a much more powerful spectroscope than Secchi’s. The
history of the particular spectroscope he employed is in one respect
specially interesting to myself, as the extension of spectroscopic
power was of my own devising before I had ever used or even seen a
powerful spectroscope. The reader is aware that spectroscopes derive
their light-sifting power from the prisms forming them. The number
of prisms was gradually increased, from Newton’s single prism to
Fraunhofer’s pair, and to Kirchhoff’s battery of four, till six were
used, which bent the light round as far as it would go. Then the idea
occurred of carrying the light to a higher level (by reflections)
and sending it back through the same battery of prisms, doubling the
dispersion. Such a battery, if of six prisms, would spread the spectral
colours twice as widely apart as six used in the ordinary way, and
would thus have a dispersive power of twelve prisms. It occurred
to me that after taking the rays through six prisms, arranged in a
curve like the letter C, an intermediate four-cornered prism of a
particular shape (which I determined) might be made to send the rays
into another battery of six prisms, the entire set forming a double
curve like the letter S, the rays being then carried to a higher level
and back through the double battery. In this way a dispersive power of
nineteen prisms could be secured. My friend, Mr. Browning, the eminent
optician, made a double battery of this kind,[13] which was purchased
by Mr. W. Spottiswoode, and by him lent to Mr. Huggins for the express
purpose of dealing with the task Secchi had set spectroscopists. It
did not, however, afford the required evidence. Huggins considered
the displacement of dark lines due to the sun’s rotation to be
recognizable, but so barely that he could not speak confidently on the
point.
There for a while the matter rested. Vögel made observations confirming
Huggins’s results relative to stellar motions; but Vögel’s instrumental
means were not sufficiently powerful to render his results of much
weight.
But recently two well-directed attacks have been made upon this
problem, one in England, the other in America, and in both cases with
success. Rather, perhaps, seeing that the method had been attacked and
was supposed to require defence, we may say that two well-directed
assaults have been made upon the attacking party, which has been
completely routed.
Arrangements were made not very long ago, by which the astronomical
work of Greenwich Observatory, for a long time directed almost
exclusively to time observations, should include the study of the sun,
stars, planets, and so forth. Amongst other work which was considered
suited to the National Observatory was the application of spectroscopic
analysis to determine motions of recession and approach among the
celestial bodies. Some of these observations, by the way, were made, we
are told, “to test the truth of Doppler’s principle,” though it seems
difficult to suppose for an instant that mathematicians so skilful as
the chief of the Observatory and some of his assistants could entertain
any doubt on that point. Probably it was intended by the words just
quoted to imply simply that some of the observations were made for the
purpose of illustrating the principle of the method. We are not to
suppose that on a point so simple the Greenwich observers have been in
any sort of doubt.
At first their results were not very satisfactory. The difficulties
which had for a long time foiled Huggins, and which Secchi was never
able to master, rendered the first Greenwich measures of stellar
motions in the line of sight wildly inconsistent, not only with
Huggins’s results, but with each other.
Secchi was not slow to note this. He renewed his objections to the new
method of observation, pointing and illustrating them by referring to
the discrepancies among the Greenwich results. But recently a fresh
series of results has been published, showing that the observers
at Greenwich have succeeded in mastering some at least among the
difficulties which they had before experienced. The measurements
of star-motions showed now a satisfactory agreement with Huggins’s
results, and their range of divergence among themselves was greatly
reduced. The chief interest of the new results, however, lay in the
observations made upon bodies known to be in motion in the line of
sight at rates already measured. These observations, though not wanted
as tests of the accuracy of the principle, were very necessary as tests
of the qualities of the instruments used in applying it. It is here and
thus that Secchi’s objections alone required to be met, and here and
thus they have been thoroughly disposed of. Let us consider what means
exist within the solar system for thus testing the new method.
The earth travels along in her orbit at the rate of about 18⅓ miles
in every second of time. Not to enter into niceties which could only
properly be dealt with mathematically, it may be said that with this
full velocity she is at times approaching the remoter planets of the
system, and at times receding from them; so that here at once is a
range of difference amounting to about 37 miles per second, and fairly
within the power of the new method of observation. For it matters
nothing, so far as the new method is concerned, whether the earth is
approaching another orb by her motion, or that orb approaching by its
own motion. Again, the plant Venus travels at the rate of about 21½
miles per second, but as the earth travels only 3 miles a second less
swiftly, and the same way round, only a small portion of Venus’s motion
ever appears as a motion of approach towards or recession from the
earth. Still, Venus is sometimes approaching and sometimes receding
from the earth, at a rate of more than 8 miles per second. Her light
is much brighter than that of Jupiter or Saturn, and accordingly this
smaller rate of motion would be probably more easily recognized than
the greater rate at which the giant planets are sometimes approaching
and at other times receding from the earth. At least, the Greenwich
observers seem to have confined their attention to Venus, so far as
motions of planets in the line of sight are concerned. The moon, as
a body which keeps always at nearly the same distance from us, would
of course be the last in the world to be selected to give positive
evidence in favour of the new method; but she serves to afford a useful
test of the qualities of the instruments employed. If when these were
applied to her they gave evidence of motions of recession or approach
at the rate of several miles per second, when we know as a matter of
fact that the moon’s distance never[14] varies by more than 30,000
miles during the lunar month, her rate of approach or recession thus
averaging about one-fiftieth part of a mile per second, discredit
would be thrown on the new method—not, indeed, as regards its
principle, which no competent reasoner can for a moment question, but
as regards the possibility of practically applying it with our present
instrumental means.
Observations have been made at Greenwich, both on Venus and on the
moon, by the new method, with results entirely satisfactory. The method
shows that Venus is receding when she is known to be receding, and
that she is approaching when she is known to be approaching. Again,
the method shows no signs of approach or recession in the moon’s case.
It is thus in satisfactory agreement with the known facts. Of course
these results are open to the objection that the observers have known
beforehand what to expect, and that expectation often deceives the
mind, especially in cases where the thing to be observed is not at all
easy to recognize. It will presently be seen that the new method has
been more satisfactorily tested, in this respect, in other ways. It may
be partly due to the effect of expectation that in the case of Venus
the motions of approach and recession, tested by the new method, have
always been somewhat too great. A part of the excess may be due to the
use of the measure of the sun’s distance, and therefore the measures
of the dimensions of the solar system, in vogue before the recent
transit. These measures fall short to some degree of those which result
from the observations made in December, 1874, on Venus in transit, the
sun’s distance being estimated at about 91,400,000 miles instead of
92,000,000 miles, which would seem to be nearer the real distance. Of
course all the motions within the solar system would be correspondingly
under-estimated. On the other hand, the new method would give all
velocities with absolute correctness if instrumental difficulties
could be overcome. The difference between the real velocities of Venus
approaching and receding, and those calculated according to the present
inexact estimate of the sun’s distance, is however much less than
the observed discrepancy, doubtless due to the difficulties involved
in the application of this most difficult method. I note the point,
chiefly for the sake of mentioning the circumstance that theoretically
the method affords a new means of measuring the dimensions of the
solar system. Whensoever the practical application of the method has
been so far improved that the rate of approach or recession of Venus,
or Mercury, or Jupiter, or Saturn (any one of these planets), can be
determined on any occasion, with great nicety, we can at once infer
the sun’s distance with corresponding exactness. Considering that
the method has only been invented ten years (setting aside Doppler’s
first vague ideas respecting it), and that spectroscopic analysis as a
method of exact observation is as yet little more than a quarter of
a century old, we may fairly hope that in the years to come the new
method, already successfully applied to measure motions of recession
and approach at the rate of 20 or 30 miles per second, will be employed
successfully in measuring much smaller velocities. Then will it give
us a new method of measuring the great base-line of astronomical
surveying—the distance of our world from the centre of the solar
system.
That this will one day happen is rendered highly probable, in my
opinion, by the successes next to be related.
Besides the motions of the planets around the sun, there are their
motions of rotation, and the rotation of the sun himself upon his
axis. Some among these turning motions are sufficiently rapid to be
dealt with by the new method. The most rapid rotational motion with
which we are acquainted from actual observation is that of the planet
Jupiter. The circuit of his equator amounts to about 267,000 miles,
and he turns once on his axis in a few minutes less than ten hours, so
that his equatorial surface travels at the rate of about 26,700 miles
an hour, or nearly 7½ miles per second. Thus between the advancing and
retreating sides of the equator there is a difference of motion in
the line of sight amounting to nearly 15 miles. But this is not all.
Jupiter shines by reflecting sunlight. Now it is easily seen that where
his turning equator _meets_ the waves of light from the sun, these are
shortened, in the same sense that waves are shortened for a swimmer
travelling to meet them, while these waves, already shortened in this
way, are further shortened when starting from the same advancing
surface of Jupiter, on their journey to us after reflection. In this
way the shortening of the waves is doubled, at least when the earth is
so placed that Jupiter lies in the same direction from us as from the
sun, the very time, in fact, when Jupiter is most favourably placed
for ordinary observation, or is at his highest due south, when the sun
is at his lowest below the northern horizon—that is, at midnight.
The lengthening of the waves is similarly doubled at this most
favourable time for observation; and the actual difference between the
motion of the two sides of Jupiter’s equator being nearly 15 miles
per second, the effect on the light-waves is equivalent to that due
to a difference of nearly 30 miles per second. Thus the new method
may fairly be expected to indicate Jupiter’s motion of rotation. The
Greenwich observers have succeeded in applying it, though Jupiter has
not been favourably situated for observation. Only on one occasion,
says Sir G. Airy, was the spectrum of Jupiter “seen fairly well,”
and on that occasion “measures were obtained which gave a result in
remarkable agreement with the calculated value.” It may well be hoped
that when in the course of a few years Jupiter returns to that part of
his course where he rises high above the horizon, shining more brightly
and through a less perturbed air, the new method will be still more
successfully applied. We may even hope to see it extended to Saturn,
not merely to confirm the measures already made of Saturn’s rotation,
but to resolve the doubts which exist as to the rotation of Saturn’s
ring-system.
Lastly, there remains the rotation of the sun, a movement much more
difficult to detect by the new method, because the actual rate of
motion even at the sun’s equator amounts only to about 1 mile per
second.
In dealing with this very difficult task, the hardest which
spectroscopists have yet attempted, the Greenwich observers have
achieved an undoubted success; but unfortunately for them, though
fortunately for science, another observatory, far smaller and of much
less celebrity, has at the critical moment achieved success still more
complete.
The astronomers at our National Observatory have been able to recognize
by the new method the turning motion of the sun upon his axis. And
here we have not, as in the case of Venus, to record merely that the
observers have seen what they expected to see because of the known
motion of the sun. “Particular care was taken,” says Airy, “to
avoid any bias from previous knowledge of the direction in which a
displacement” (of the spectral lines) “was to be expected,” the side of
the sun under observation not being known by the observer until after
the observation was completed.
But Professor Young, at Dartmouth College, Hanover, N.H., has done much
more than merely obtain evidence by the new method that the sun is
rotating as we already knew. He has succeeded so perfectly in mastering
the instrumental and observational difficulties, as absolutely to be
able to rely on his _measurement_ (as distinguished from the mere
recognition) of the sun’s motion of rotation. The manner in which he
has extended the powers of ordinary spectroscopic analysis, cannot very
readily be described in these pages, simply because the principles on
which the extension depends require for their complete description a
reference to mathematical considerations of some complexity. Let it be
simply noted that what is called the diffraction spectrum, obtained
by using a finely lined plate, results from the dispersive action
of such a plate, or _grating_ as it is technically called, and this
dispersive power can be readily combined with that of a spectroscope
of the ordinary kind. Now Dr. Rutherfurd, of New York, has succeeded
in ruling so many thousand lines on glass within the breadth of a
single inch as to produce a grating of high dispersive power. Availing
himself of this beautiful extension of spectroscopic powers, Professor
Young has succeeded in recognizing effects of much smaller motions
of recession and approach than had before been observable by the new
method. He has thus been able to measure the rotation-rate of the sun’s
equatorial regions. His result exceeds considerably that inferred from
the telescopic observation of the solar spots. For whereas from the
motion of the spots a rotation-rate of about 1¼ mile per second has
been calculated for the sun’s equator, Professor Young obtains from his
spectroscopic observations a rate of rather more than 1⅖ mile, or about
300 yards per second more than the telescopic rate.
If Young had been measuring the motion of the same matter which is
observed with the telescope, there could of course be no doubt that
the telescope was right and the spectroscope wrong. We might add a
few yards per second for the probably greater distance of the sun
resulting from recent transit observations. For of course with an
increase in our estimate of the sun’s distance there comes an increase
in our estimate of the sun’s dimensions, and of the velocity of the
rotational motion of his surface. But only about 12 yards per second
could be allowed on this account; the rest would have to be regarded
as an error due to the difficulties involved in the spectroscopic
method. In reality, however, the telescopist and the spectroscopist
observe different things in determining by their respective methods
the sun’s motion of rotation. The former observes the motion of the
spots belonging to the sun’s visible surface; the latter observes the
motion of the glowing vapours outside that surface, for it is from
these vapours, not from the surface of the sun, that the dark lines
of the spectrum proceed. Now so confident is Professor Young of the
accuracy of his spectroscopic observations, that he is prepared to
regard the seeming difference of velocity between the atmosphere and
surface of the sun as real. He believes that “the solar atmosphere
really sweeps forward over the underlying surface, in the same way
that the equatorial regions outstrip the other parts of the sun’s
surface.” This inference, important and interesting in itself, is far
more important in what it involves. For if we can accept it, it follows
that the spectroscopic method of measuring the velocity of motions in
the line of sight is competent, under favourable conditions, to obtain
results accurate within a few hundred yards per second, or 10 or 12
miles per minute. If this shall really prove to be true for the method
now, less than ten years after it was first successfully applied,
what may we not hope from the method in future years? Spectroscopic
analysis itself is in its infancy, and this method is but a recent
application of spectroscopy. A century or so hence astronomers will
smile (though not disdainfully) at these feeble efforts, much as we
smile now in contemplating the puny telescopes with which Galileo and
his contemporaries studied the star-depths. And we may well believe
that largely as the knowledge gained by telescopists in our own
time surpasses that which Galileo obtained, so will spectroscopists
a few generations hence have gained a far wider and deeper insight
into the constitution and movements of the stellar universe than the
spectroscopists of our own day dare even hope to attain.
I venture confidently to predict that, in that day, astronomers will
recognize in the universe of stars a variety of structure, a complexity
of arrangement, an abundance of every form of cosmical vitality, such
as I have been led by other considerations to suggest, not the mere
cloven lamina of uniformly scattered stars more or less resembling our
sun, and all in nearly the same stage of cosmical development, which
the books of astronomy not many years since agreed in describing. The
history of astronomical progress does not render it probable that the
reasoning already advanced, though in reality demonstrative, will
convince the generality of science students until direct and easily
understood observations have shown the real nature of the constitution
of that part of the universe over which astronomical survey extends.
But the evidence already obtained, though its thorough analysis may
be “_caviare_ to the general,” suffices to show the real nature of
the relations which one day will come within the direct scope of
astronomical observation.
_THE NEW STAR WHICH FADED INTO STAR-MIST._
The appearance of a new star in the constellation of the Swan in the
autumn of 1876 promises to throw even more light than was expected on
some of the most interesting problems with which modern astronomy has
to deal. It was justly regarded as a circumstance of extreme interest
that so soon after the outburst of the star which formed a new gem
in the Northern Crown in May, 1866, another should have shone forth
under seemingly similar conditions. And when, as time went on, it
appeared that in several respects the new star in the Swan differed
from the new star in the Crown, astronomers found fresh interest in
studying, as closely as possible, the changes presented by the former
as it gradually faded from view. But they were not prepared to expect
what has actually taken place, or to recognize so great a difference
of character between these two new stars, that whereas one seemed
throughout its visibility to ordinary eyesight, and even until the
present time, to be justly called a star, the other should so change as
to render it extremely doubtful whether at any time it deserved to be
regarded as a star or sun.
Few astronomical phenomena, even of those observed during this century
(so fruitful in great astronomical discoveries), seem better worthy
of thorough investigation and study than those presented by the two
stars which appeared in the Crown and in the Swan, in 1866 and 1876
respectively. A new era seems indeed to be beginning for those
departments of astronomy which deal with stars and star-cloudlets on
the one hand, and with the evolution of solar systems and stellar
systems on the other.
Let us briefly consider the history of the star of 1866 in the first
place, and then turn our thoughts to the more surprising and probably
more instructive history of the star which shone out in November, 1876.
In the first place, however, I would desire to make a few remarks
on the objections which have been expressed by an observer to whom
astronomy is indebted for very useful work, against the endeavour to
interpret the facts ascertained respecting these so-called new stars.
M. Cornu, who made some among the earliest spectroscopic observations
of the star in Cygnus, after describing his results, proceeded as
follows:—“Grand and seductive though the task may be of endeavouring
to draw from observed facts inductions respecting the physical state of
this new star, respecting its temperature, and the chemical reactions
of which it may be the scene, I shall abstain from all commentary and
all hypothesis on this subject. I think that we do not yet possess
the data necessary for arriving at useful conclusions, or at least at
conclusions capable of being tested: however attractive hypotheses may
be, we must not forget that they are outside the bounds of science,
and that, far from serving science, they seriously endanger its
progress.” This, as I ventured to point out at the time, is utterly
inconsistent with all experience. M. Cornu’s objection to theorizing
when he did not see his way to theorizing justly, is sound enough;
but his general objection to theorizing is, with all deference be it
said, sheerly absurd. It will be noticed that I say theorizing, not
hypothesis-framing; for though he speaks of hypotheses, he in reality
is describing theories. The word hypothesis is too frequently used in
this incorrect sense—perhaps so frequently that we may almost prefer
sanctioning the use to substituting the correct word. But the fact
really is, that many, even among scientific writers, when they hear
the word hypothesis, think immediately of Newton’s famous “hypotheses
non fingo,” a dictum relating to real hypotheses, not to theories. It
would, in fact, be absurd to suppose that Newton, who had advanced,
advocated, and eventually established, the noblest scientific theory
the world has known, would ever have expressed an objection to
theorizing, as he is commonly understood to have done by those who
interpret his “hypotheses non fingo” in the sense which finds favour
with M. Cornu. But apart from this, Newton definitely indicates what he
means by hypotheses. “I frame no hypotheses,” he says, “_for whatever
is not deduced from phenomena is to be called an hypothesis_.” M.
Cornu, it will be seen, rejects the idea of deducing from phenomena
what he calls an hypothesis, but what would not be an hypothesis
according to Newton’s definition: “Malgré tout ce qu’il y aurait de
séduisant et de grandiose à tirer de ce fait des inductions, etc., je
m’abstiendrai de tout commentaire et de toute hypothèse à ce sujet.” It
is not thus that observed scientific facts are to be made fruitful, nor
thus that the points to which closer attention must be given are to be
ascertained.