Of the atmosphere.
In descending to the lower strata of the atmosphere, the
reader should be informed that we here restrict the term to
the portion of ambient air next the Earth, which receives
vapours and exhalations, and increasingly reacts the rays
of light. Ptolemy had already remarked that the light of
the stars underwent a change of direction in the course of
passing through this terrestrial envelope; and every
observer is aware of its darkening effect, especially under
high magnifying power. It therefore follows, that the
pervading consequence of angular altitude is a paramount
condition to be considered before the amateur expends much
time upon objects near the horizon, wherever he may happen
to be located. On this account the diatribe was uttered
against low stars (see ante, page 46); though perhaps an
occasional dip among the remote regions of the South may
hereafter yield comparisons in aid of further
investigations of the phenomena - visible and invisible -
of vapours.
A conviction of this will very soon come home to the
intelligent. observer, who will shape a course accordingly,
trimming agreeably to his means and intentions; and his
judgment must guide him in selecting objects for a
systematic attack. Not even will a tropical climate free
the spectator from these tremulous deceptions. On the
contrary, the absence of aqueous vapour may be perceived by
anybody to diminish the apparent steadiness of objects, in
proportion to the amount of heat and distance from the
eye.
Meantime, as it is the most able discussion of the point
that I am aware of, I will here extract my son’s
generalization of the argument for our climate from the
twelfth volume of the Edinburgh Astronomical Observations,
page 466. [*83]
ATMOSPHERIC EFFECTS.
Argument of C.P. Smyth.
“When we look at the broad discs of either noon or sun in the act of rising or setting, the colouring effects of the atmosphere are plainly visible on them, as every painter knows right well; and the same effects we might expect to witness on a star seen in a telescope under the same small angle of altitude; but, when we do actually look into such an instrument, the adventitious colour of the region for a surface of sensible size is generally overpowered by the prismatic effect on the mere point presented by a star, which is thereby converted almost entirely into a spectrum of red on one side and blue on the other. Of course the observer attempts to eliminate these prismatic tints from, or rather takes no note of them at all in, the true cosmical colour of the centre of the stellar image; but does he succeed in eliminating both these and every other colouring effect of the atmosphere ? To ascertain this point, I have taken all the stars observed on Tenerife in south declination, and, having strengthened them from some observations by Mr. J. W. Grant at Calcutta, and by Sir T. Maclear at the Cape of Good Hope, have compared then in the matter of colour, with the best observations of north and middle Europe; and then, dividing them into three groups, of three different degrees of more and more southerly position, we have as below:-
Influence of atmosphere at low altitude.
| Star | Components | Magnitude | European Colour | Teneriffe and Indian Colour |
| 113 P. Ceti | A | 7 | Yellow | Strong yellow |
| B | 9 | Blue | Warm grey | |
| 146 P. Ceti | A | 6.5 | Yellow | Yellow |
| B | 9 | Violet | Pale violet | |
| 32 Eridani | A | 5 | Orange | Orange |
| B | 7 | Greenish | Greenish | |
| 5 Aquilæ | A | 7 | White | Pale yellow |
| B | 8 | Blue lilac | Bluish | |
| ψ1 Aquarii | A | 4 | Orange yellow | Cadmium Yellow |
| B | 4.5 | Blue | Blue |
| Star | Components | Magnitude | European Colour | Teneriffe and Indian Colour |
| ν Serpentis | A | 4.5 | Greenish | Bluish |
| B | 9.0 | Coppery | Warm blue | |
| 185 P. Antinoi | A | 9 | Yellow | White |
| B | 10 | Blue | White | |
| C | 12 | Violet | Blue | |
| 186 P. Antinoi | A | 7.5 | Whitish | Yellow |
| B | 9 | Blue | Blue | |
| α2 Capricorni | A | 3 | Yellow | Yellow |
| B | 16 | Blue | Blue | |
| τ1 Aquarii | A | 6 | White | Light yellow |
| B | 9.5 | Violet | Pale violet | |
| 94 Aquarii | A | 6 | Orange | Yellowish |
| B | 8.5 | Faint lilac | Greenish blue | |
| 107 Aquarii | A | 6 | White | Pale yellow |
| B | 7.5 | Bluish | White |
| Star | Components | Magnitude | European Colour | Teneriffe and Indian Colour |
| α Scorpii | A | 1 | Fiery red | Orange red |
| B | 11 | Emerald | Pure blue | |
| 39 Ophiuchi | A | 5.0 | Orange | Pale yellow |
| B | 7.5 | Bluish | Faint blue | |
| α2 Piscis Austr. | A | 1 | Reddish | White |
| B | 9.5 | Dusky blue | Blue |
Atmospheric effects perceptible.
“Now, from the first of these lists, it would appear that all its five pairs of stars are coloured to European, almost precisely as to more southern, observers; and that, in so far, European observations of colour are quite safe down to 10 degrees below the celestial equator. [*85] But in the second list, or from 10 to 20 degrees below the equator, three out of its seven stars show unmistakable symptoms of atmospheric colouration - yellowish being changed into orange, bluish into greenish, and white into blue. While in the third list, or from 20° to 30° south declination, every star there shows, and in each of its components, the same effect to an increased degree - white being turned into reddish, pale yellow into orange, orange red into fiery red, and pure blue into bluish green, or dusky blue, as depending doubtless on variations of the lower atmosphere at the several times of observation.
Safe limits for colour observations on stars.
“Hence there can be hardly a doubt but that, in the
important physical question of the real colours of stars,
no European observations should be received below the
fifteenth degree of south declination; and that, if
powerful spectrum analysis be employed, the range should
not exceed the fifth degree, or just so far beyond the
equator as may give a southern observatory, attending to
the southern stars, a few common objects of observation for
index differences.
“Meanwhile, with regard to the Teneriffe and Elchies
stars, it is most satisfactory to find, after throwing out;
the cases of atmospheric disturbance, that the rest are, on
the whole, so very trustworthy and similar to both the
‘Cycle’ and F. W. Struve’s colours, that,
if a case of particular divergence be therein found, it is
capable of being referred to a physical change of the
star's real colour; almost as confidently indeed as an eye
observation of change of brightness may be assigned to a
real change of ‘magnitude;’ and, consequently,
renders the exact date of the chromatic phenomenon,
necessary accompaniments to its description.”[*86]
On Nebulae.
ALTHOUGH the cause is utterly unknown, and in the present
stage of human cognoscence appears to be inscrutable, it is
surmised that the exceptional bodies designated
Nebulæ have a connection with double-stars (see
Arago’s Popular Astronomy, book xi. chapter
xxiv.) while, as to colours, I have noticed in them
pale tints of white, creamy white, yellow, green, and blue.
It therefore follows that these incomprehensible but
palpable evidences of Omnipotent power and design are not
unnecessarily hauled in and appended to our dissertation
upon Sidereal Chromatics.
It will be recollected by all who are really concerned
about tile matter, that, when the wondrous revelations of
Lord Rosse were communicated to the public, certain buzzing
popinjays, who hang about, and obstruct the avenues to the
temple of science, vociferously proclaimed that the Nebular
Theory had received its coup de grace from the castle at
Parsonstown. Now this crude conceit was assuredly not
imbibed from his Lordship's statement, he having most
pointedly said, that “now, is as has always been the
case, an increase of instrumental power has added to the
number of clusters at the expense of the nebulæ
properly so called; still it would be very unsafe to
conclude that such will always be the case, and thence to
draw the obvious inference that all nebulosity is but the
glare of stars too remote to be separated by tile utmost
power of our instruments.”
In the Speculum Harlwellianztm (pages 111-114) I
gave my fully considered opinion on this head, showing tile
actual state of the question, and advocating that planetary
nebulæ diffuse patches of we know not what emanating
light, arm all the “island universes” so
profusely scattered in the abysses of space, should be
competently watched for ages. Now those “Thoughts on
the Nebular Hypothesis” were written in the year
1860, as an addendum to what I had already published on the
same subject in my Cycle of Celestial Objects. It
was therefore with pleasure, while this, the last sheet, is
passing through the press, that I received a letter from my
[*87] unflagging
friend Mr. William Huggins, in which he thus announces the
conclusion arrived at front the masterly experiments lately
made at Tulse Hill: “I fancy you will be interested
in the result of some observations I have recently made on
the spectra of some of the nebulæ. I have
obtained evidence, which I believe will be accepted as
satisfactory, that certain of the nebulæ(at present
my list contains five PLANETARY nebulæ and the
annular one in Lyra) are certainly, NOT clusters
of stars. They are probably enormous masses of glaiding
or ammoniacal gas containing comparatively small quantities
of matter condensed into the liquid or solid state. The
observations are now at the printer's, and I hope within a
fortnight to send you a copy of them. After the opinions
which you have published, thinking you must be greatly
interested in the matter, I have ventured to trouble you
with this note in anticipation.”
So enchanting a vista of successive discoveries has or late
been thrown open to us, through following up the
long-neglected unravelling properties of the Prism, and
applying the delicate yet unequivocal test to the heavenly
bodies. that a few familiar explanations of the spectra of
so experienced a hand as Mr. Huggins may be acceptable to
many an amateur-gazer who is now endeavouring on fine
nights to interpret those brilliants. In expounding the
subject, he thus popularly expresses himself:-
Mr. Huggin’s explanation.
“The dark spaces due to interference are supposed to be produced by the action of light upon light- but the dark lines of the spectrum by the absorptive action of vapours or gases upon light. In the ease of interference according to the undulatory theory, when two waves of homogeneous light from the same source, and proceeding by two different routes a little unequal in length, meet in opposite phases of an undulation, they destroy each other’s motion, and so there results a calm in the luminiferous ether. When no waves dash upon the retina, the eye is without stimulus, and this calm we call darkness.
“The present theory of the nature and origin of the dark lines of the solar spectrum is quite recent. In 1855 Balfour Stewart published a paper on the law of exchanges in radiant heat. In the following year the subject was taken up by Kirchoff, who extended the theory to light. The conclusion at which he arrived may be thus stated :- When any substance is heated or is rendered luminous, rays of certain and definite degrees of refrangibility are given out by it, whilst the same substance has also the power of absorbing rays or these identical refrangibilities. Thus the light of burning sodium or glaiding, sodium vapour, cannot get through the vapour of sodium, though this vapour is wholly powerless to absorb or quench light of any refrangibility other than that which sodium vapour emits when heated till it is luminous. In this way the double line D of the solar spectrum can be experimental produced.
“How is light absorbed by gas or vapour? The theory is as follows:- The atoms [*88] of a gas being freer to move than those of a liquid or a solid, are capable of swinging or vibrating at certain definite rates only. These atoms have also the power of intercepting the waves which were excited by atoms swinging at the same rates, and these only. Thus atoms, the motions of which are suitable for the emission of red light, will stop red light, and so on. [17]
“The maps accompanying my paper show the several distinct rates or vibration corresponding to the motions of the gaseous atoms of the elements described. Each line by its place in the spectrum indicates a definite wave-length, just as each note of a piece of music indicates a definite rate of vibration of the air. These maps may be termed the light-songs or the elements written down in notes. Absorption is therefore according to this theory a transference of motion from the ether to the material particles immersed in it.
“Thus it appears when the light emitted from the solid or liquid photosphere of the sun has to traverse its atmosphere, crowded with vapours of different substances, each vapour stops its own group of lines of light, and so the original light reaches us lessened by the aggregate of all the groups of all the vapours. Now, when this light is spread out by the prism, the dark doings of these vapours are revealed, and dark lines or dark spaces show the places where tile light has been intercepted.”
Chemical substances.
Such are the reasonable expectations of observational and experimental science, as connected with the constitution of those substances which optical analysis afforded the means of discovering in the heavenly bodies. A reading of Mr. Huggins's statement to the Royal Society on the “Spectra of some of the Chemical Elements” will show the care and labour involved in constructing the Spectroscope apparatus with which he scrutinized gold, silver, thallium, cadmium, lead, tin, bismuth, antimony, potassium, arsenic, palladium, lithium, strontium, platinum, tellurium, osmium, rhodium, iridium, manganese, chromium, cobalt, nickel, and iron. In fine: if the pestilent earth-annihilating prophets who have recently frightened some old women, of both sexes, out of their wits, will but permit our globe to roll noiselessly on its axis for another century astronomy will be found in possession of many realities which, in 1864, are only to be classed among the numerous desiderata of Uranology.
β Cygni
In employing various eyes and instruments upon this fine double β Cygni star, I was, of course, desirous of securing the evidence afforded by the powerful achromatic telescope at Greenwich. But; it was only as this sheet is being concluded - considerably after the eleventh hour - that I received the reply; yet it is of that; interest to the question, that I here append it as received.
Royal Observatory, Greenwich, London, 8.E.
1864, September 29, evening.
Mr Dear Sir,Various circumstances have impeded the examination of β Cygni, but; here I send it at last. You may depend upon the result as accurately; it has been carefully referred to an accurate standard.
I give it, on the other leaf, in the words in which it was given to me.I am, my dear Sir,
Yours most truly,
G. B. AIRY. Admiral W. H. Smyth.
Observations by Mr. J. Carpenter.
“The large star is bright yellow, about the colour of
that part of the solar spectrum situate at a point about
one-eighth of the distance between Frauenhofer’s D
and E, reckoning from D towards E. (Sensibly the same
colour as the flame of a hand-lamp fed with Colza oil).
“The small star is pale blue, about the colour of
that part of the spectrum which is crossed by the F.
(Sometimes it was suspected that the light of this star had
a slightly greenish tinge.)
“These comparisons were made on two evenings with a
drawing of the spectrum by M. Chevreul, in his
‘Exposè d'un, Moyen de dèfinir et de
nommer les Couleurs,’ and this drawing was afterwards
compared with the solar spectrum itself at the points used
for the star comparisons, and was found to be accurate.
“When the eye-piece was pushed within the proper
focus, the contrast between the colours of the discs seemed
more striking than when the proper focus was used; and,
when the eye-piece was pulled out beyond the proper
focus, the contrast seemed less striking.
“The colours were most strongly contrasted with low
powers, as they were also when the image was viewed without
any eye-piece. With higher powers the colours became a
little more nearly similar, the yellow star seeming to
retain its colour, but the blue becoming a little yellowish
in its centre.”
The ending.
One word in conclusion. With all my admiration of the marvellous and extensive power of Chemistry in disintegrating the nature and properties of the elements of matter, I really trust it will not be exerted among the Celestials to the disservice or detriment of measuring agency; and this I hope for the absolute maintenance of GEOMETRY, DYNAMICS, and pure ASTRONOMY.
[17] “When two waves meet in the relative position shown in No. 1 in consequence of the opposite direction of their motion they are both destroyed. But when two
waves meet in the relative position shown in No. 2, they co-operate, and there is a double amount of light.
See also Professor Tyndall on "Heat as a mode of Motion.”
(NO
ADS!)