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Admiral Smyth’s — Sidereal ChromaticsPART II. (a.) (cont.)DOUBLE STAR COLOURS CONTINUEDChromatic difficulties. Under our present practice [*36], various difficulties arc presented, for the designation of hues uttered in mere parlance by several persons often means the same tints in different words, and these will not always quadrate with the chromatic language of photologists. The wonder, however, is not so great that, without a due nomenclature, we should differ so much from each other, and even from ourselves at different dates, but rather that, with such an unorganized practice, so many instances should coincide relatively. In many cases the difference of colour in the components of a double star are real; but when they are merely complementary, the fainter of the two may possibly be a white star which appears to have the colour complementary to that of its more brilliant companion. This is in consequence of a well-understood law of vision, by which the retina of the eye being excited by light of a particular colour, is rendered insensible to less intense light of the same hue, — so that the complement of the real light of the fainter star finds the retina more sensible to it, than to the ray which is identical in colour with the brighter star; and the impression of the complementary tint accordingly prevails. But the accurate perception of the colour of a celestial body often depends as much on the condition of the eye when the object is seen, as upon the object itself; and possibly the achromatism of the object-glass, which, being adapted to the solar spectrum may not be suitable to the spectrum of a star, ought to be taken into account; as well as a nice adjustment of the eyepiece, to lead to a discrimination between real and illusive appearances. The powers of colours in contrasting with each other, agree with their correlative powers of light and shade; and such are to be distinguished from their powers individually on the eye, which are those of light alone. It may assist the memory of the inexperienced observer, to remind him that the primary colours and their complementaries are in these relations — RED GREEN
|
| Colour | Parts of an Inch. [7] |
| Red | 0.00002582 |
| Orange | 0.00002319 |
| Yellow | 0.00002270 |
| Green | 0.00002073 |
| Blue | 0.00001912 |
| Indigo | 0.00001690 |
| Violet | 0.00001572 |
Now, though this by itself may say nothing with respect to the rapidity with which each undulation may be transmitted, it renders the probability of such a difference extremely great; and, though that difference be so very small that there is no hope of ever being able to make it manifest to any scientific apparatus of overt the most delicate description, yet, on account of the vast remoteness of the stars, the effect may become at length, very sensible. For, although the [*42] difference in the rate of propagation by the waves of each ray may be the smallest conceivable quantity, yet, if that different rate be kept up during the whole of the one thousand years that we suspect must be occupied by the light of some of the stars in reaching us, notwithstanding that it may travel on the average one hundred and ninety-two thousand miles in a second, it is manifest that, after continuing to grow during so great a length of time, a very decided effect, may at last be produced.
Assumed instance.
If a new star suddenly appears in any part of the sky, the rays of light immediately travel off to announce the fact everywhere, and to us amongst the number of other orbs; and it matters not whether the light consist in the emission of particles, or the propagation of waves of different orders, as many of Arago’s “couriers” as there are different colours in the spectrum are sent off with the intelligence; and, if one is able to accomplish the great intervening distance between the star and us in a less space of time than the others, and so arrive before them, we shall see the star of that colour first, say violet. In that case the next to arrive would be the yellow, and then arriving and mixing with the blue, already come, would make the star change from pure blue to green; while the positive red, arriving last of all, and joining themselves to the existing green, would at length make the star appear white; and, if it preserved the same lust. re, it would ever after continue white.
Reminder.
But be it recollected that, in the ideas evoked by the discrepancies of colour-estimates, I am only throwing out suggestions, not advocating an hypothesis; still it must be admitted that variations of colour ought to accompany variations of brightness, though such variation of hue has not hitherto been detected in some stars that notably vary in splendour. With this confession, we will proceed in the vision thus conjured up, and return to the celestial body in white; only reminding the reader, that little is correctly understood of light in its causes [*43] and principles of existence, and that Bacon has told us — revtè scire est per causas scire. [8]
Argument resumed.
If the above-cited star be shown for only an instant of time as
an electric spark, then we might see it varying through each of the
different colours, blue, yellow, and red, separately and distinctly.
Allowing that, for example, the blue ray was to traverse the space
between the star and ourselves in three years, the yellow in three
years and one week, and the red in three years and two weeks, and
supposing the above to apply only to the central portion of each
coloured ray, which should gradually vary with filaments of
different velocities so as to join insensibly with those of the
neighbouring one; then, three years after the striking of this
stellar spark, we should see a blue star appear in the sky, and lash
for one week; then the star would appear yellow during another week,
and red during another; after which it would be lost altogether. Or
if there be actual separations between the different colours, as is
more than hinted at by the discovery of the black bands in the
spectrum, then the star, after appearing of one colour, might even
disappear for a time before the next colour began to arrive.
Again, if a star which has existed for ages be on a sudden
extinguished, the rays last emitted will be the couriers to announce
the fact; and, supposing the star to have been white, three years
afterwards (in the above particular example), the last of the blue
rays having arrived before the last of the others, the blue will be
deficient in the star, and from white it will become orange; after a
week all the yellow ones will have come in, and the star will be
red; and, when the final rays of this colour have arrived, it will
totally disappear. [*44]
But if the star shines permanently, and has so shone from time
immemorial, then, whatever might be the difference of time elapsing
between the blue and red rays shot from the star at the same instant
reaching us, we should see the star white; for blue and yellow and
red rays of different dates of emission would all be reaching our
eyes together.
Practical exemplification.
This case can be exemplified by looking through a prism at a white surface of unlimited extent and equal brightness, when it will be seen as white as before; for the multitudinous spectra formed by all the component points of the whole surface overlaying each other, the red of one coming to the blue and yellow of others, will form white light as completely as if the three colours of one point be concentrated together again. Here was Goethe’s error: he gazed at a white wall through a prism, and, finding it white still, kicked at Newton&8217;s theory to produce an absurd one of his own. But had he looked at the edges of the wall — which is a similar case to the birth or death of a star — he would have seen the blue half of the spectrum on one side, and the, red on another: everything, in fact, with a sensible breadth will have coloured borders, blue on one side and red on the other. If one part of the wall, however, be brighter than another, the strong blue of that portion thrown on to the fainter red of another, will give flint a bluish tinge, and vice versa; and so with the stars; if their brightness should alter, or, in the common though singularly erroneous parlance, their magnitudes vary, the strong blue of a bright, epoch arriving with faint red of a dull period, will make blue appear to us as the predominating colour; will cause indeed the star’s light to appear decidedly blue at one time, and, mutatis mutandis, red at another, although all the while the star’s colour may not really have altered at all; but may have been really, and would have appeared to observers close by, as white as ever, varying only in quantity and not in quality. Real alterations in colour may doubtless occur, yet evidently may also often be only consequences of alterations in brightness, which may be brought about by many regular and periodical phenomena, and certainly [*45] do not require the introduction of any such startling reason, as the conflagration that was lugged in to explain the tints through which the variable star of 1572 passed, as it gradually died out. Of the sky, where it had so suddenly appeared a few months previously. Of this, at least, we may be certain, that there are periodical variations in the brightness of the stars, and that some alteration of colour should thereby be produced; but whether to a sensible extent or not, is only to be determined by experiment. β Persei has been selected by Arago as a measurable instance For testing this matter by observation, because it changes so very rapidly in brightness in a short space of time; but, though he did not succeed in detecting any alteration of colour we must not despair; for, while on the one hand his means of determining the colour seem to have had no sensible degree of exactness, it is easily possible to assume such a difference of velocities for the various coloured rays of the star, and such a distance for them to traverse, as should completely annihilate the expected good effect of the quickness and frequent recurrence of the changes in this particular star. Many other stars might indeed be picked out where the natural circumstances are more promising, while further steps towards the perfection of the means of observation, would allow of many more still being made subservient to the inquiry.
How to observe.
The failures made heretofore may therefore be regarded in the
same light as those in the oldest inquiry of finding the parallax of
the fixed stars, viz. not as reasons For leaving off, but for trying
again more energetically, more extensively, and with more accurate
means than before; and, although I may not be prepared just at
present to describe any perfectly satisfactory method of
observation, stilt, as some amateurs desirous of pursuing the
subject may like to see such hints as lily experience has
incidentally given rise to, presented in some rather more practical
form, I have thrown them together as follows:
In any method of determining colours of stars, three possible
sources of error have to be met: 1. The state of the [*46] atmosphere generally at the time in
altering the colour of all the stars above the horizon; 2. The
effect of altitude in varying on different stars the apparent colour
produced by the atmosphere; and 3. The effect on the eye of the
necessary quantity of some sort or other of artificial light, for
the propose of writing down or examining the dimensions of the
instrument, the face of the clocks, &c. &c.
First source.
The first can only be culminated by extensive observations of a number of stars, especially circumpolar ones, all throughout the year. Although the colours of some stars may vary in a small number of months, weeks, or even days, the mean of them all may be considered to be safely depended on for a tolerably constant quantity; and each star should be examined and tested for its colour every night, by comparison with the mean of all the rest; and where any decided variation appears to be going on through the year, that star should at once be excluded from the standard list, and its difference from the mean of the others stated as its colour for each night’s observation.
Second source.
The second source of error is to be met by observations of the
same star through a large part of its path from rising to
culminating, or a number of stars of known colour at various
altitudes, combined with a correction something similar to that for
refraction, as varying in a proportion not far from the tangent of
the zenith distance; and which would consequently require the
altitude of every body observed to be carefully noted, as a
decidedly necessary element introducing the observations.
Low stars, however, should be eschewed, and each observer should
confine himself as far as possible to his zenith stars; for, in
addition to the low ones being so much fainter to him, than to one
to whom they are vertical, and in addition to the colouring and
absorbing effect of the atmosphere increasing so excessively low
down on the horizon, the envelope acts so strongly there as a prism,
that, combined with the bad definition prevailing, I have sometimes
seen a large star of a [*47] really
white colour appear like a blue and red handkerchief fluttering in
the wind: the blue and red about as intense and decided as they
could well be. This shows the extreme importance of noting not only
the altitude of the star, which determines also the degree of
prismatic effect, but of distinguishing in the observation any
difference between the upper and lower parts of the star. In the Sun
and Moon, bodies of very sensible breadth, — this effect is
not so evident; the surface will still be white or coloured
uniformly by the atmosphere, and the upper and lower borders will
alone show the prismatic colours, half on one edge and the other
half on the other, as in the case off the white wall mentioned
above; but the star under discussion, being merely a point of light,
is wholly acted on, and exhibits as complete a spectrum as could be
contrived without any of the white or self-compensating intermediate
portion.
Combined with this is the colouring effect of the object-glass, and
any deficiency in its achromaticity; though these, being nearly the
same on all the stars, will not affect the difference observed: yet
the latter quality of the eyepiece will be of more consequence,
unless the star be brought very rigorously into the centre of the
field off view, and kept there the whole time that it is under
observation. A well achromatized eyepiece should be specifically
used, and its assigned magnifying power always recorded.
Third source.
The third difficulty may be best counteracted by using one eye
for the field of the telescope, and the other for writing down,
&tc.; having the artificial lights used for these purposes as
faint and making the, them as white as possible, with various other
little practical details which will best occur to each observer.
We then come to the grand difficulty: viz. the manner in vehicle the
colour is to be determined; the methods are two: first, by the
senses; second, by instrumental means. The first is that which has
been employed hitherto, and will doubtless still be the only method
employed for a considerable time by amateurs; and, though so very
vague, yet may — by the education and the practice of the
senses, combined with the corrections [*48]
above considered — be carried to considerable
perfection: still the education must be much more systematic, and
the practice much more constant, than they have hitherto been. Nor
will the pursuit be altogether unfruitful, even if it only relieves
science by thereby proving a negative; but to the zealous aspirant
there is a hopeful guerdon, because much of the theory of the
universe may be finally revealed by this elegant and difficult
element.
Some certain standard of colours must be kept and constantly
referred to: the colours of precious stones have been used for this
purpose; but, though very properly in one point of view, as being by
their brightness more comparable to stars than ordinary pigments,
yet astronomers in general have not much acquaintance with anything
so valuable and costly; and, if they had, would find that the colour
of each star is not certainly to be defined by the jewel, i.e. that
under the same name many different colours may be found; and
different observers will therefore be giving the same name to stars
not resembling each other; in addition to which there is not a
sufficient range or colours amongst the precious stones to meet all
the cases which occur in nature in the heavens, and they neither
admit of being mixed, to form varieties of colour, nor of being
modified, to show graduations in their own colour; a most important
defect. These qualities, however, are possessed by the water colours
of the present day; the greater part of them are very permanent, and
the others, which are not so, are capable of being prepared fresh
and fresh; the number of colours moreover is great, the combinations
that may be formed of them almost endless; and graduations of each
may be made, from nearly white to all but black. Not only must a
scale of them be had in possession, and frequently referred to, but
it must be made and remade by the observer, as a mode of impressing
the colours on his memory; and, unless he can carry them in his
mind, he need not attempt the chromatic observation of stars; for as
he cannot see the star and his scale of colour at the same time and
side by [*49] side, the estimate of the
star depends entirely on the accidentals of memory. [9]
A word to the tyro.
Not to be too dogmatical, however, with the willing neophyte in his outset, we may observe, that, though the aptitude thus recommended may be troublesome to attain, it is approach aisle by opening trenches; and nature has kindly provided many individuals with the requisites for receiving delicate impressions by the senses. Now the source of colours is acknowledged to be light: each primary tone being surrounded by its harmonizing secondary, which is again bordered by its tertiary; and the perceptive faculty at ones distinguishing their several shades is an endowment of the most pleasing power, whether exerted in a passion for stars, flowers, and mundane finery — or in contributing to render the painter’s art impressive to the imagination, and delightful to the eye. While therefore a spectator is able to enjoy the sensitive perception of the varied gradations of hues and tints, he need not involve himself in the vexata questio as to colours being material or not — whether entities or individualities. That they are not yet really reducible to a single principle is no reason why they should not be used most comprehensibly, and every advance must be duly encouraged for the results that may ensue.
ENDNOTES
[6] The notion of male light being imparted hy the Sun, and female light by the Moon, is as old as the hills. Pliny, in his CYCLOPÆDIA (lib. ii. esp. 100 and 101,) mentions it as a condition, “which we have been taught;” and he details the influences of the masculine and feminine stars. here, probably, Milton imbibed the hint to which I alluded in the Cycle ( I. page . 301)—
“Other suns,
perhaps,
With their attendant moons, thou wilt descry,
Communicating male and female light.”
[7] These lengths of an undulation lead to tile astounding inference, that, on viewing a red object, the membrane of the eye trembles at tho rate of 480,000,000,000,000 of times in every two beats of a seconds’ pendulum! The researches and discoveries of Huyghens, Young, Malus, Fresnel, Arago, Poisson, Airy, Wheatstone, and others, bare rendered the hypothesis of an undulatory propagation of light almost a demonstrated truth. “It is a theory,” says Herschel, “which, if not founded in nature, is certainly one of the happiest fictions that the genius of man has yet invented to group together natural phenomena, as well as the most fortunate in the support it has received from all classes of new phenomena, which at their discovery seemed in irreconcilable opposition to it. It is, in fact, all its applications and details, one succession felicities; inasmuch that we may almost be induced to say, if it be not true, it deserves to be.”
[8] In a letter of March 10th, 1860, the Master of Trinity, Dr. Whewell, says — “If I was writing a review of your splendid volume (Spectulum Hartwellianum), I should of course try to find some fault in it by way of showing my acuteness : and I should say— ‘At page 324, we read Bacon has told us — rectrè scire est percauses scire. We may remark that it was Aristotle who said this in Greek, and his followers in Latin. We may add that it is not a maxim which has done much good in science, for the first step is to learn the laws of the phenomena. The cause afterwards if we can: but, if we cannot, we have still learned something.”
[9] Chromatography is not near perfection as the power of the eye and state of art would lead us suppose that Mr. Chevreul’s beautiful work on Colours, which has appeared since the above was printed, will yield a useful standard of tints for astrometry, as well as for manufacture, so as to afford an easy and ready reference.
Last Update : 26th April 2017
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