By G.E. LUMSDEN.
[Footnote 1: Paper read before the Astronomical and Physical
Society of Toronto, Canada, April 18, 1891.]
These are the palmiest days in the eventful history of physical and
observational astronomy. Along the whole line of professional and
amateur observation substantial progress is being made, but in certain
new directions, and in some old ones, too, the advance is very rapid.
As never before, public interest is alive to the attractions and value
of the work of astronomers. The science itself now appeals to a
constituency of students and readers daily increasing in numbers and
importance. Evidence of this gratifying fact is easily obtained. There
is at the libraries an ever-growing demand for standard astronomical
works, some of them by no means intended to be of a purely popular
character. Some of the most influential and conservative magazines on
both sides of the Atlantic now find it to be in their interest to
devote pages of space to the careful discussion of new theories, or to
the results of the latest work of professional observers. Even the
daily press in some cities has caught the infection, if infection it
may be called. There are in New York, Philadelphia, St. Louis, and
other centers of population on this continent leading newspapers
which, every week or so, publish columns of original matter
contributed by writers evidently able to place before their readers in
an attractive form articles dealing accurately, and yet in a popular
vein, with the many-sided subject of astronomy. In scientific matters
generally, there is abroad in this and other countries a spirit of
inquiry, never more apparent than at the present time.
Readers and thinkers may, no doubt, be numbered by thousands. So far,
however, as astronomy is concerned, the majority of readers and
thinkers is composed of non-observers, most of whom believe they must
be content with studying the theoretical side of the subject only.
They labor under the false impression that unless they have telescopes
of large aperture and other costly apparatus, the pleasures attaching
to practical work are denied them. The great observatories, to which
every intelligent eye is directed, are, in a measure, though
innocently enough, responsible for this. Anticipation is ever on
tiptoe. People are naturally awaiting the latest news from the giant
refracting and reflecting telescopes of the day. Under these
circumstances, it may be that the services rendered, and capable of
being rendered, to science by smaller apertures may be overlooked,
and, therefore, I ask to be permitted to put in a modest plea for the
common telescope. What little I shall have to say will be addressed to
you more for the purpose of arousing interest in the subject than for
communicating to you any information of a novel or special character.
When making use of the term "common telescope," I would like to be
understood as referring to good refractors with object glasses not
exceeding three or three and one-half inches in diameter. In some
works on the subject telescopes as large as five inches or even five
and one-half inches are included in the description "common," but
instruments of such apertures are not so frequently met with in this
country as to justify the classing of them with smaller ones, and,
perhaps, for my purpose, it is well that such is the fact, for the
expense connected with the purchase of first rate telescopes increases
very rapidly in proportion to the size of the object glass, and soon
becomes a serious matter. Should ever the larger apertures become
numerous on this continent, let us hope it shall be found to have been
as one of the results of societies like this, striving to make more
popular the study of astronomy.
It is not by any means proposed to inflict upon you a history of the
telescope, but your indulgence is asked for a few moments while
reference is made to one or two matters connected with its invention,
or, rather, its accidental discovery and subsequent improvement.
The opening years of the seventeenth century found the world without a
telescope, or, at least, such an instrument as was adapted for
astronomical work. It is true that long years before, Arabian and some
other eastern astronomers, for the purpose, possibly, of enabling them
to concentrate their gaze upon celestial objects and follow their
motions, had been accustomed to use a kind of tube consisting of a
long cylinder without glasses of any kind and open at both ends. For
magnifying purposes, this tube was of no value. Still, it must have
been of some kind of service, or else the first telescopes, as
constructed by the spectacle makers, who had stumbled upon the
principle involved, were exceedingly sorry affairs, for, soon after
their introduction, the illustrious Kepler, in his work on "Optics,"
recommended the employment of plain apertures, without lenses, because
they were superior to the telescope on account of their freedom from
refraction.
But as soon as the principle by which distant objects could,
apparently, be brought nearer the eye became known and its value
recognized by philosophers, telescopes ceased to be regarded as toys,
and underwent material improvements in the hands of such men as
Galilei, and, later, even of Kepler himself, Cassini, Huyghens, and
others. Galilei's first telescope magnified but three times, and his
best not much above thirty times. If I comprehend aright what has been
written upon the subject, I am justified in saying that this little
instrument in my hand, with an aperture of one inch and one-quarter,
and a focus, with an astronomical eye-piece, of about ten inches, is a
better magnifier than was Galilei's best. With it I can see the moons
of Jupiter, some spots on the sun, the phases of Venus, the
composition, in some places, of the Milky Way, the seas, the valleys,
the mountains, and, when in bold relief upon the terminator, even some
of the craters and cones of the moon. Indeed, I am of opinion I can
see even more than he could, for I can readily make out a considerable
portion of the Great Nebula in Orion, some double stars, and enough of
the Saturnian system to discern the disk of the planet and see that
there is something attached to its sides.
For nearly one hundred and fifty years all refracting telescopes
labored under one serious difficulty. The images formed by them were
more or less confused by rainbow tints, due to the bending, or
refracting, by the object glass of the rays of light. To overcome this
obstacle to clear vision, and also to secure magnification, the focal
lengths of the instruments were greatly extended. Telescopes 38, 50,
78, 130, 160, 210, 400, and even 600 feet long were constructed. I
can, however, find nothing on record indicating that the object
glasses of these enormously attenuated instruments ever exceeded in
diameter two and one-half inches. Yet, with unwieldy and ungainly
telescopes, nearly always defining badly, wonders were accomplished by
the painstaking and indomitable observers of the time.
In 1658, Huyghens, using a telescope twenty-three feet long and two
and one-third inches in diameter, with a power of 100, solved the
mystery of Saturn's rings, which had resisted all of Galilei's efforts
as well as his own with a shorter instrument, though he had discovered
Titan, Saturn's largest moon, and fixed correctly its period of
revolution at sixteen days. Fifteen years later, Ball, with a
telescope thirty-eight feet long, discovered the principal division in
the rings. Ten years still later, Cassini, with an instrument twenty
feet long and an object glass two and one-half inches in diameter,
rediscovered the division, which was named after him, rather than
after Ball, who had taken no pains to make widely known his discovery,
which, in the meantime, had been forgotten. Though we have no record,
there is no doubt that the lamented Horrocks and Crabtree, in England,
in 1639, with glasses no better than these, watched with exultant
emotions the first transit of Venus ever seen by human eyes.
In 1722, Bradley, with a telescope 223¼ feet long, succeeded in
measuring the diameter of the same planet. Yet Grant assures us that,
in spite of all their difficulties, such was the industry of the
astronomers that when, at the commencement of this century, it became
possible to construct larger refracting telescopes, there was nothing
to be discovered that could have been discovered with the means at
their disposal. So far as we now know, a good three-inch telescope,
nay, a first-rate two inch one, will show far more than our
great-grandfathers ever saw, or dreamed of seeing, with their
refractors.
Toward the middle of the seventeenth century the reflecting telescope
had been so much improved as nearly to crowd out its refracting rival,
but, just as its success seemed to be assured, Dollond, working along
lines partially followed up by Hall, found a combination of lenses by
which the chromatic aberration of the refractor could be very
perfectly corrected. While Dollond's invention was of immense value,
it remained that flint object glasses larger than two and one-half
inches in diameter could not, for some years, be manufactured, but
about the opening of the nineteenth century, Guinand, a Swiss,
discovered a process of making masses of optical flint glass
sufficiently large as to admit of the construction from them of
excellent lenses of sizes gradually increasing as time and
experimenting went on. The making of three-inch objectives, achromatic
and of short focus, wrought a revolution in telescopes and renewed the
demand for refractors, though prices, as compared with those of the
present day, were very great. But improvement was succeeded by
improvement. Larger and still larger objectives were made, yet
progress was not so rapid as not to justify Grant, in 1852, in
declaring to be a "munificent gift" the presentation, about 1838, to
Greenwhich Observatory, of a six and seven-tenths object glass alone,
and so it was esteemed by Mr. Airy, the astronomer royal. Improvement
is still the order of the day, and, as a result of keen competition,
very excellent telescopes of small aperture can be purchased at
reasonable prices. Great telescopes are enormously expensive, and will
probably be so until they are superseded by some simple invention
which shall be as superior to them as they are to the "mighty"
instruments which, from time to time, caused such sensations in the
days of Galilei, Cassini, Huyghens, Bradley, Dollond, and those who
came after them.
But, notable as are the services rendered to science by giant
telescopes, it remains that by far the greater bulk of useful work has
been done by apertures of less than twelve inches in diameter. Indeed,
it may be asserted that most of such work has been done by instruments
of six inches or less in size. After referring with some detail to
this, Denning tells us that "nearly all the comets, planetoids, double
stars, etc., owe their detection to small instruments; that our
knowledge of sun spots, lunar and planetary features is also very
largely derived from similar sources; that there is no department
which is not indebted to the services of small telescopes, and that of
some thousands of drawings of celestial objects, made by observers
employing instruments from three to seventy-two inches in diameter, a
careful inspection shows that the smaller instruments have not been
outdone in this interesting field of observation, owing to their
excellent defining powers and the facility with which they are used."
Aperture for aperture, the record is more glorious for the "common
telescope" than for its great rivals. Let us for a moment recall
something of what has been done with instruments which may be embraced
under the designation "common" as such a statement may serve to remove
impressions that small telescopes are but of little use in
astronomical work.
In his unrivaled book, Webb declares that his observations were
chiefly made with a telescope five and one-half feet long, carrying an
object glass of a diameter of three and seven-tenths inches. The
instrument was of "fair defining quality," and one has but to read his
delightful pages in order to form an idea of the countless pleasures
Webb derived from observation with it. Speaking of it, he says that
smaller ones will, of course, do less, especially with faint objects,
but are often very perfect and distinct, and that even diminutive
glasses, if good, will, at least, show something never seen without
them. He adds: "I have a little hand telescope twenty-two and
one-quarter inches long, when fully drawn out, with a focus of about
fourteen inches, and one and one-third inches aperture; this, with an
astronomical eye-piece, will show the _existence_ of sun spots, the
mountains in the moon, Jupiter's satellites and Saturn's ring." In
another place, speaking of the sun, he says that an object glass of
only two inches will exhibit a curdled or marbled appearance over the
whole solar disk, caused by the intermixture of spaces of different
brightness. And I may add here that Dawes recommends a small aperture
for sun work, including spectroscopic examinations, he himself, like
Mr. Miller, our librarian, preferring to use for that purpose a four
inch refractor.
As you know, the North Star is a most beautiful double. Its companion
is of the ninth order of magnitude, that is, three magnitudes smaller
than the smallest star visible to the naked eye on a dark night. There
was a time when Polaris, as a double, was regarded as an excellent
test for a good three inch telescope; that is any three inch
instrument in which the companion could be seen was pronounced to be
first-class. But so persistently have instruments of small aperture
been improved that that star is no longer an absolute test for three
inch objectives of fine quality, or any first-rate objective exceeding
two inches for which Dawes proposed it as a standard of excellence, he
having found that if the eye and telescope be good, the companion to
Polaris may be seen with such an aperture armed with a power of
eighty. As a matter of fact, Dawes, who was, like Burnham, blessed
with most acute vision, saw the companion with an instrument no larger
than this small one in my hand--one inch and three-tenths. Ward saw it
with an inch and one-quarter objective, and Dawson with so small an
aperture as one inch. T.T. Smith has seen it with a reflector stopped
down to one inch and one-quarter, while in the instrument still known
as the "great Dorpat reflector," it has been seen in broad daylight.
This historic telescope has, I believe, a twelve inch object glass,
but the difficulty of seeing in sunshine so minute a star is such that
the fact may fairly be mentioned here.
Another interesting feature is this. Objects once discovered, though
thought to be visible in large telescopes only, may often be seen in
much smaller ones. The first Herschel said truly that less optical
power will show an object than was required for its discovery. The
rifts, or canals, in the Great Nebula in Andromeda is a case in point,
but two better illustrations may be taken from the planets. Though
Saturn was for many years subjected to most careful scrutiny by
skilled astronomers using the most powerful telescopes in existence,
the crape ring eluded discovery until November, 1850, when it was
independently seen by Dawes, in England, and Bond, in the United
States. Both were capital observers and employed excellent instruments
of large aperture, and it was naturally presumed that only such
instruments could show the novel Saturnian feature. Not so. Once
brought to the attention of astronomers, Webb saw the new ring with
his three and seven-tenths telescope and Ross with an aperture not
exceeding three and three-eighths in diameter. Nay, I am permitted to
say that a venerable member of this society made drawings of it with a
three inch refractor. With a two inch objective, Grover not only saw
the crape ring, but Saturn's belts, as well, and the shadow cast by
the ball of the planet upon its system of rings. Titan, Saturn's
largest moon, is merely a point of light as compared with the planet,
as it appears in a telescope, yet it has been seen, so it is said,
with a one inch glass. The shadow of this satellite, while crossing
the face of Saturn, has been observed by Banks with a two and
seven-eighths objective. By hiding the glare of the planet behind an
occulting bar, some of Saturn's smallest moons were seen by Kitchener
with a two and seven-tenths aperture and by Capron with a two and
three-fourths one. Banks saw four of them with a three and
seven-eighths telescope, Grover two of them with a three and
three-quarter inch, and four inches of aperture will show five of
them, so Webb says. Rhea, Dione and Tethys are more minute than
Japetus, yet Cassini, with his inferior means, discerned them and
traced their periods. Take the instance of Mars next. It was long
believed that Mars had no satellites. But in 1877, during one of the
highly favorable oppositions of that planet which occur but once in
about sixteen years, the able Hall, using the great 26 inch refractor
at Washington, discovered two tiny moons which had never been seen
before. One of these, called Deimos, is only six miles in diameter,
the other, named Phobos, is only seven, and both are exceedingly close
to the primary and in rapid revolution. The diameter of these
satellites is really less than the distance from High Park, on the
west of Toronto, to Woodbine race course, on the east of the city. No
wonder these minute objects--seldom, if ever, nearer to us than about
forty millions of miles--are difficult to see at all. Newcomb and
Holden tell us that they are invisible save at the sixteen year
periods referred to, when it happens that the earth and Mars, in their
respective orbits, approach each other more nearly than at any other
time. But once discovered, the rule held good even in the case of the
satellites of Mars. Pratt has seen Deimos, the outermost moon, with an
eight and one-seventh inch telescope; Erek has seen it with a seven
and one-third inch achromatic; Trouvellot, the innermost one, with a
six and three-tenths glass, while Common believes that any one who can
make out Enceladus, one of Saturn's smallest moons, can see those of
Mars by hiding the planet at or near the elongations, and that even
our own moonlight does not prevent the observations being made. It
chances for the benefit of observers, in the northern hemisphere
especially, that one of the sixteen year periods will culminate in
1893, when Mars will be most advantageously situated for close
examination. No doubt every one will avail himself of the opportunity,
and may we not reasonably hope that scores of amateur observers
throughout the United States and Canada will experience the delight of
seeing and studying the tiny moons of our ruddy neighbor?
And so I might proceed until I had wearied you with illustrations
showing what can be done with telescopes so small that they may fairly
be classed as "common," Webb says that such apertures, with somewhat
high powers, will reveal stars down to the eleventh magnitude. The
interesting celestial objects more conspicuous than stars of that
magnitude are sufficiently numerous to exhaust much more time than any
amateur can give to observing. Indeed, the lot of the amateur is a
happy one. With a good, though small, telescope, he may have for
subjects of investigation the sun with his spots, his faculæ, his
prominences and spectra; the moon, a most superb object in nearly
every optical instrument, with her mountains, valleys, seas, craters,
cones, and ever-changing aspects renewed every month, her occupations
of stars, her eclipses, and all that; the planets, some with phases,
and other with markings, belts, rings, and moons with scores of
occupations, eclipses and transits due to their easily observed
rotation around their primaries; the nebulæ, the double, triple and
multiple stars with sometimes beautifully contrasted colors, and a
thousand and one other means of amusing and instructing himself.
Nature has opened in the heavens as interesting a volume as she has
opened on the earth, and with but little trouble any one may learn to
read in it.
I trust it has been shown that expensive telescopes are not
necessarily required for practical work. My advice to an intending
purchaser would be to put into the objective for a refractor, or into
the mirror for a reflector, all the money he feels warranted in
spending, leaving the mounting to be done in the cheapest possible
manner consistent with accuracy of adjustment, because it is in the
objective or in the mirror that the _value_ of the telescope alone
resides. In the shops may be found many telescopes gorgeous in
polished tubes and brass mountings which, for effective work, are
absolutely worthless. On this subject, I consulted the most eminent of
all discoverers of double stars, an observer who, even as an amateur,
made a glorious reputation by the work done with a six inch telescope.
I refer to Mr. S.W. Burnham, of the Lick Observatory, who, in reply,
kindly wrote: "You will certainly have no difficulty in making out a
strong case in favor of the use of small telescopes in many
departments of important astronomical work. Most of the early
telescopic work was done with instruments which would now be
considered as inferior to modern instruments, in quality as well as in
size. You are doubtless familiar with much of the amateur work, in
this country and elsewhere, done with comparatively small apertures.
_The most important condition is to have the refractor_, whatever its
size may be, _of the highest optical perfection_, and then the rest
will depend on the zeal and industry of the observer." The italics are
mine.
Incidentally, it may be mentioned that much most interesting work may
be done even with an opera glass, as a few minutes' systematic
observation on any fine night will prove. Newcomb and Holden assure us
that "if Hipparchus had had even such an optical instrument, mankind
need not have waited two thousand years to know the nature of the
Milky Way, nor would it have required a Galilei to discover the phases
of Venus or the spots on the sun." To amplify the thought, if that
mighty geometer and observer and some of his contemporaries had
possessed but the "common telescope," is it not probable that in the
science of astronomy the world would have been to-day two thousand
years in advance of its present position?
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