The Victorian Omnium-Gatherum
On Monday next, March 15, will occur the only eclipse of the sun which will be nearly total in England during the remainder of the present century. Throughout a strip of England it will be annular, that is, the moon will hide all the sun except a narrow ring all round the edge. And this time the ring will be extremely narrow. In London, nearly the whole of the sun will be eclipsed, but not annularly: nearly 98 per cent. of the sun's diameter will be covered at one minute past one. The line of annular eclipse enters England close to Lyme Regis at a few minutes before 1 p.m., and passes near Yeovil, Castle Cary, Westbury, Swindon, Lechlade, Witney, Brackley, Towcester, Northampton, Wellingborough, Oundle, Crowland, Holbeach, and out at the Wash at about 2 minutes after 1 p.m. Mr Hind says:—
"The central line near to which the greatest degree of darkness and the more imposing phenomena are to be expected will cross the Great Western Railway, as above stated, close to the station at Swindon, which will accordingly be the most accessible point in this direction for those residents in the metropolis who may wish to view the eclipse to the best advantage. It will traverse the main line of the London and North-Western about a mile from the Blisworth station; and, leaving Peterborough four miles to the south-east, will cross the Great Northern Railway near its junction with the East Lincolnshire and Midland lines. The respective companies might probably add to their revenues by affording facilities for the conveyance of the public to the above points on the morning of the 15th of March. Swindon will, perhaps, be the more eligible position, as the magnitude of the eclipse will be somewhat greater there than higher up the central line. To the unprofessional observer I would suggest that he will find his advantage in leaving telescopes at home. The phenomena which require their aid are not certain to occur in this eclipse, and while he is looking for them he would, undoubtedly, lose the best view of those far grander appearances on the earth and in the heavens which have especially rivetted the observer's attention in great eclipses of the sun. A few coloured glasses varying in depth of shade will be useful in watching the successive phases of the eclipse, but he will need no other assistance. A perusal of some popular account of the appearances attending the totality will probably enable him to see more than he would do if entirely unacquainted therewith; he will then know what to look for, and will be less likely to be disconcerted by the almost supernatural aspect of everything around him. It cannot be too well understood that it is only upon or very near to the central line that we are to anticipate the most striking effects."
We heartily concur in the advice to take no telescope. But if there should be any persons who do not believe they have seen an astronomical phenomenon unless they have peeped through an artificial eye with which they have never learnt to see, we recommend a common opera-glass, of not quite so high a power as is used by those who are very particular about the length of a dancer's nose. This, with a few pieces of coloured glass, will be ample for the purpose. And let no one turn up his own nose at an opera-glass : remember that it is the old telescope of all, the one with which the "Tuscan artist" did first "descry new worlds," We make another quotation from Mr. Hind:—
"When two-thirds, or rather more, of the sun's diameter are covered by the moon, or when the sun has assumed the figure presented by the moon three or four days before the change, a decided alteration in the colour of the landscape will be remarked; a gradually deepening yellow tinge will creep over it, and about the same time has generally commenced that period of unusual stillness of nature which is frequently a marked characteristic of the absence of sunlight. Ten minutes or thereabouts previous to the greatest eclipse the pale or azure blue of the sky will change to violet or purple, the horizon will begin to close in on every side of the spectator, and shortly after the heavens will appear to descend upon him. This apparent descent of the sky struck me as one of the most astonishing and imposing effects of the totality in 1851: indeed, on that occasion it was truly appalling. For two or three minutes at the time of the greatest obscuration the planet Venus and several of the brightest stars will probably come into view, while everything around the observer will have assumed that unnatural gloomy appearance which has never failed to induce feelings of awe. Objects will then appear tinged with dull olive or purple; the clouds, if favourably placed for the effect, will seem to be almost in contact with him, and the black moon projected on the face of the sun, and surrounded by a brilliant halo, will appear to be hardly more than a hundred yards distant. However a person may have prepared himself for the phenomena of a great eclipse it is not unlikely that his self-possession may desert him when the grandeur of the scene is before him; and I am inclined to attribute to this circumstance that want of accordance in the descriptions of some of the appearances during totality upon which several writers; not eye-witnesses) have considered themselves justified in advancing certain theories by no means reconcileable with the impressions of actual observers. A curious appearance has repeatedly presented itself shortly before the commencement of totality and soon after its termination, or when the solar crescent has a scarcely appreciable breadth. It consists in the rapid passage of dark and light waves over the ground or along walls, and was particularly remarked in the south of France during the eclipse of 1842, and at several stations in Prussia in 1851. In the metropolis the eclipse will be very large, though not annular; 978-1000ths of the sun's diameter will be covered by the moon at 1h. 1m. P.M., or the solar crescent will then present a breadth of less than 45 seconds of arc. This corresponds to the appearance of the sun about 1m. 10s. previous to the commencement of totality in the eclipse of July, 1851, and from experience on that occasion I entertain little doubt that there will be a very considerable degree of gloom in London; not, perhaps, what would be termed darkness, but rather partaking of that unnatural shade which invariably accompanies the total phase. Stars or planets may be seen, particularly if their positions are known to the observer beforehand. The sky will appear to close in and deepen almost into violet, and the aspect of things generally may be changed."
It would be useful to many of our readers who want to know the eclipsed sun, first to make a little more acquaintance with the uneclipsed body. Let them watch the sun through a dark glass for about a quarter of an hour on the day immediately preceding the eclipse.
The following suggestions for observation of the eclipse are offered by the Astronomer Royal:—
I. Observations not requiring Instruments.
1. As the eclipse advances, it is desirable to obtain some
notion or measure of the degree of darkness.
2. At what distance from the eye can a book or paper, exhibiting type of different sizes, be read ?
3. Hold up a lighted candle nearly between the sun and your eye. At how many sun-breadths' distance from the sun can the flame be seen ?
4. If you are in an elevated position, remark the changes of colour and appearance of the surrounding objects in the landscape.
5. If you see the spots of light formed by the intersecting shadows of the boughs of trees, remark whether they exhibit the luneform of the sun.
6. When the annulus is formed you will probably observe it with a darkened glass; but you are particularly requested to devote one instant (as early as possible) to the verification of this point—viz. when the annular sun is viewed with the naked eye does it appear an annulus or a fully illuminated disc?
II. Optical, Astronomical, and Solar-Physical Observations requiring the Use of Instruments,
7. As the eclipse advances, estimate (on the image seen in
the telescope) the comparative intensity of the sun's light near the
centre of his disc and near his limb.
8. For the more critical observations it is desirable that the power of your telescope should be so low as to give you an easy view of the whole breadth of the sun.
9. Remark irregularities on the moon's limb.
10. As the cusps become very sharp, remark whether they are irregular. For this, and for all the observations near the annular phase, it is necessary that you be provided either with a graduated prismatic shade, or with a succession of shades of different intensity, and that you instantly select the shade which is most agreeable to your eye.
11. Remark whether the sun's light extends beyond the intersection of the limbs of sun and moon, so as to make the moon's limb visible beyond that intersection. For this purpose the bright parts of the sun must be put out of the field of view, and the shade must be withdrawn.
12. As the annularity approaches and is formed, remark whether Baily's Beads and Strings are formed ; whether first formed at points corresponding to large inequalities of the moon's limb; whether they surround the moon; how they form and break. Only an instant can be given to this observation. It is of the utmost importance to be assured that your vision at the instant immediately preceding, especially of the moon's inequalities, is very distinct.
13. Remark, as one of the most important observations of the eclipse, whether any red flames are seen on the sun's limb. For this purpose you must withdraw the shade, if you are on the annular track, the instant after formation of the annulus, if you are not on the annular track, as soon as the eye can bear the sun. On the annular track the whole line must be rapidly scrutinized; and when the ring breaks the still illuminated part must be put out of the field, and the moon's dark limb must be surveyed. At places not on the annular track this plan (namely, to exclude the illuminated portion of the disc from the field, and to survey the moon's dark limb) must be followed throughout. It seems not improbable that the best chance of seeing red flames will be obtained at places not on the annular track.
14. At the breaking up of the annulus, look for Baily's Beads as before.
15. Do not attempt any record during or near the annularity. Endeavour to impress observations on your memory as well as you can. If you have an assistant seated at a table with a chronometer and writing materials, you may give him signals for the register of time; but you must connect the phenomena with the time afterwards.
16. A good sextant observer may obtain valuable observations for correction of the lunar tables by measuring the intervals between the points of the bright cusps. The observations will require great nerve, and will be difficult; but where most difficult they will be most valuable.
17. It seems doubtful whether any valid photographic record can be made, on account of the extreme rapidity of the change of appearances. Thus, in the neighbourhood of London, the line of cusps will change from the vertical to the horizontal position in about three minutes of time.
18. If you have a doubly-refracting prism it will be desirable to make observations on the polarization of the light from the sun's limb. For this purpose, when the lune is narrow, place the prism so as to separate the two images transversely to the limb, and remark which image is brighter. Turn the prism 180 degrees round the visual ray, and repeat the observation. Remark carefully the positions of the prism. The prism may be used with the naked eye, or with the telescope, according to the amount of its angular separation of images.
III. Meteorological Observations.
19. For change in intensity of solar radiation, observations
with the actinometer or the black-bulb thermometer should be kept up
during the eclipse. The latter are most trustworthy when the bulb is
inclosed in an exhausted glass sphere.
20. The barometer should be repeatedly observed.
21. The thermometer should be frequently observed, and the general feelings of cold should be noted.
22. Observations of humidity are very important. They should be made by the use either of Daniell's dew-point instrument, or of the wet-bulb thermometer.
G. B. AIRY.
An Easy Method of obtaining Photographic Records of the approaching Solar Eclipse.
Permit me to suggest to your readers a very simple and inexpensive means of obtaining large photographs of the eclipse of the 15th inst.
When the sun shines perpendicularly on a double concave lens, the greater number of rays pass through, suffering refraction and dispersion; others, however, are reflected from the two surfaces: the first surface —acting like a concave speculum —reflects them back again to a focus, and forms an image of the sun, varying in size according to the depth of curvature. The clearness of this image is, however, interfered with by rays which are reflected from the second, interior, surface of the lens, which, acting in this case like a convex mirror, reflects and disperses them on and around the image formed in the focus of the first surface. The reflection from this second surface can be easily got rid of by grinding it with fine emery, and then coating it with a solution of black sealing-wax in spirit, or with any other black varnish.
My proposition then is, to procure a double concave spectacle-glass, sufficiently shallow to reflect an image of the sun at least several feet off, so as to get it of a good size (No. 00, for instance); roughen and black one side, as above mentioned, and mount it at the end of a tube or box, so that a beam of sunlight may pass in at one end, fall nearly perpendicular on the surface of the lens, and thus be reflected back again to a little on one side of the hole by which it entered, where must be an arrangement for holding the focusing-glass, collodion plate-holder, &c. The size of aperture required to be placed in front of the lens, so as to reduce the photographic intensity to within manageable bounds, is a matter for experiment alone to decide.
I will not trespass on your space by entering into details of the arrangement, as they are such as would readily suggest themselves to any one,—if not at the time of first putting together the apparatus, at all events at one of the experimental trials which should be made previously, in order to become thoroughly familiar with the working of the instrument when the time for action arrives.
Some advantages of this plan are, the great diminution of light and heat occasioned by reflection from a glass surface;—this will be appreciated by any one who has tried his hand at helio-photography; and the slight expense, as an outlay of a few pence, and a little mechanical ingenuity, will put any one in possession of an instrument capable of giving a very fair image of the sun of any desired size. No clockwork will be required, as the exposure will be but a very small fraction of a second. The distortion of the image arising from the surface not being truly ground is not so great as would be imagined,—as with an ordinary very shallow concave spectacle glass I have obtained excellent projections of the solar spots; there is, besides, no difficulty about the focus, as the visual and chemical focus of reflected images are coincident.
Of course, where access can be had to a telescope, by all means let it be converted into a camera for the purpose; but my object in writing this is to show how any one acquainted with photography may easily obtain large and valuable photographs of this rare and important astronomical phenomenon.
I am, &c., WILLIAM CROOKES,
Secretary to the Photographic Society. 1, New Coventry Street, Piccadilly, March 4.
Published on 13 March 1858 in The Athenaeum Journal of Literature, Science and the Fine Arts.
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Optical Phenomena Observed During the Eclipse (from Punch)