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ference which divides the light into concentric rings.' One could almost wish for the opportunity of showing Young how literally his words are fulfilled, and how beautifully his theory is illustrated, by these artificial circular rainbows. For here the space within the primaries is swept by concentric supernumerary bands, coloured like the rainbow, and growing gradually narrower as they retreat from the primary. These spurious bows, as they are sometimes called,1 which constitute one of the most splendid illustrations of the principle of interference, are separated from each other by zones of darkness, where the light waves, on being added together, destroy each other. I have counted as many as eight of these beautiful bands, concentric with the true primary. The supernumeraries are formed next to the most refrangible colour of the bow, and therefore occur within the primary circle. But in the secondary bow, the violet, or most refrangible colour, is on the outside; and, following the violet of the secondary, I have sometimes counted as many as five spurious bows. Some notion may be formed of the intensity of the primary, when the secondary is able to produce effects of this description.

On

An extremely handy spray-producer is that employed to moisten the air in the Houses of Parliament. A fillet of water, issuing under strong pressure from a small orifice, impinges on a little disk, placed at a distance of about one-twentieth of an inch from the orifice. striking the disk, the water spreads laterally, and breaks up into exceedingly fine spray. Here also I have used the spray-producer both singly and in groups, the latter arrangement being resorted to when showers of special breadth and density were required. In regard to primaries, secondaries, and supernumeraries, extremely brilliant effects have been obtained with this form of spray-producer. The quantity of water called upon being much less than that required by the rose, the fillet-and-disk instrument produces less flooding of the locality where the experiments are made. In this latter respect, the steam spray is particularly handy. A puff of two seconds' duration suffices to bring out the bows, the subsequent shower being so light as to render the use of waterproof clothing unnecessary. In other cases, the inconvenience of flooding may be avoided to a great extent by turning on the spray for a short time only, and then cutting off the supply of water. The vision of the bow being, however, proportionate to the duration of the shower, will, when the shower is brief, be evanescent. Hence, when quiet and continued contemplation of all the phenomena is desired, the observer must make up his mind to brave the rain.11

In one important particular the spray-producer last described commends itself to our attention. With it we can operate on sub

10 A term, I confess, not to my liking.

11 The rays which form the artificial bow emerge, as might be expected, polarised from the drops.

stances more costly than water, and obtain rainbows from liquids of the most various refractive indices. To extend the field of experiment in this direction, the following arrangement has been devised: A strong cylindrical iron bottle, wholly or partly filled with the liquid to be experimented on, is tightly closed by a brass cap. Through the cap passes a metal tube, soldered air-tight where it crosses the cap, and ending near the bottom of the iron bottle. To the free end of this tube is attached the spray-producer. A second tube passes also through the cap, but ends above the surface of the liquid. This second tube, which is long and flexible, is connected with a larger iron bottle, containing compressed air. Hoisting the small bottle to a convenient height, the tap of the larger bottle is carefully opened, the air passes through the flexible tube to the smaller bottle, exerts its pressure upon the surface of the liquid therein contained, drives it up the other tube, and causes it to impinge with any required degree of force against the disk of the spray-producer. From this it falls in a fine rain. A great many liquids, including coloured ones,12 have been tested by this arrangement, and very remarkable results have been obtained. I will confine myself here to a reference to two liquids, which commend themselves on account of their cheapness and of the brilliancy of their effects. Spirit of turpentine, forced from the iron bottle, and caused to fall in a fine shower, produces a circular bow of extraordinary intensity and depth of colour. With paraffin oil or petroleum a similar effect is obtained.

Spectrum analysis, as generally understood, occupies itself with atomic, or molecular, action, but physical spectrum analysis may be brought to bear upon our falling showers. I asked myself whether a composite shower-that is to say, one produced by the mingled spray of two or more liquids-could not be analysed and made to declare its constituents by the production of the circular rainbows proper to the respective liquids. This was found to be the case. In the ordinary rainbow the narrowest colour-band is produced by its most refrangible light. In general, the greater the refraction, the smaller will be the bow. Now, as spirit of turpentine and paraffin are both more refractive than water, I thought it probable that in a mixed shower of water and paraffin, or water and turpentine, the smaller and more luminous circle of the latter ought to be seen within the larger circle of the former. The result was exactly in accordance with this anticipation. Beginning with water, and producing its two bows, and then allowing the turpentine to shower down and mingle with the water, within the large and beautifully coloured water-wheel, the more richly coloured circle of the turpentine makes its appearance. Or, beginning with turpentine, and forming its concentrated iris; on turning on the water-spray, though to the eye the shower seems 12 Rose-aniline, dissolved in alcohol, produces a splendid bow with specially broad supernumeraries.

absolutely homogeneous, its true character is instantly declared by the flashing out of the larger concentric aqueous bow. The water primary is accompanied by its secondary close at hand. Associated, moreover, with all the bows, primary and secondary, are the supernumeraries which belong to them; and a more superb experimental illustration of optical principles it would be hardly possible to witness. It is not the less impressive because extracted from the simple combination of a beam of light and a shower of rain.

In the Philosophical Transactions for 1835, the late Colonel Sykes gave a vivid description of a circular solar rainbow, observed by him in India, during periods when fogs and mists were prevalent in the chasms of the Ghâts of the Deccan.

It was during such periods that I had several opportunities of witnessing that singular phenomenon, the circular rainbow, which, from its rareness, is spoken of as a possible occurrence only. The stratum of fog from the Konkun on some occasions rose somewhat above the level of the top of a precipice forming the north-west scarp of the hill fort of Hurreechundurghur, from 2,000 to 3,000 feet perpendicular, without coming over upon the table-land. I was placed at the edge of the precipice just without the limits of the fog, and with a cloudless sun at my back at a very low elevation. Under such a combination of favourable circumstances, the circular rainbow appeared quite perfect, of the most vivid colours, one half above the level on which I stood, the other half below it. Shadows in distinct outline of myself, my horse, and people appeared in the centre of the circle as a picture, to which the bow formed a resplendent frame. My attendants were incredulous that the figures they saw under such extraordinary circumstances could be their own shadows, and they tossed their arms and legs about, and put their bodies into various postures, to be assured of the fact by the corresponding movements of the objects within the circle; and it was some little time ere the superstitious feeling with which the spectacle was viewed wore off. From our proximity to the fog, I believe the diameter of the circle at no time exceeded fifty or sixty feet. The brilliant circle was accompanied by the usual outer bow in fainter colours.

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Mr. E. Colborne Baber, an accomplished and intrepid traveller, has recently enriched the Transactions' of the Royal Geographical Society by a paper of rare merit, in which his travels in Western China are described. He made there the ascent of Mount O-an eminence of great celebrity. Its height is about 11,000 feet above the sea, and it is flanked on one side by a cliff a good deal more than a mile in height.' From the edge of this cliff, which is guarded by posts and chains, you look into an abyss, and if fortune, or rather the mists, favour you, you see there a miracle, which is thus described by Mr. Baber:

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Naturally enough it is with some trepidation that pilgrims approach this fearsome brink, but they are drawn to it by the hope of beholding the mysterious apparition known as the 'Fo-Kuang,' or 'Glory of Buddha,' which floats in mid-air, half-way down. So many eye-witnesses had told me of this wonder, that I could not doubt; but I gazed long and steadfastly into the gulf without success, and came away disappointed, but not incredulous. It was described to me as a circle of brilliant and many-coloured radiance, broken on the outside with quick flashes

and surrounding a central disc as bright as the sun, but more beautiful. Devout Buddhists assert that it is an emanation from the aureole of Buddha, and a visible sign of the holiness of Mount O.

Impossible as it may be deemed, the phenomenon does really exist. I suppose no better evidence could be desired for the attestation of a Buddhist miracle than that of a Baptist missionary, unless, indeed, it be, as in this case, that of two Baptist missionaries. Two gentlemen of that persuasion have ascended the mountain since my visit, and have seen the Glory of Buddha several times. They relate that it resembles a golden sun-like disc, enclosed in a ring of prismatic colours more closely blended than in the rainbow. . . . The missionaries inform me that it was about three o'clock in the afternoon, near the middle of August, when they saw the meteor, and that it was only visible when the precipice was more or less clothed in mist. It appeared to lie on the surface of the mist, and was always in the direction of a line drawn from the sun through their heads, as is certified by the fact that the shadow of their heads was seen on the meteor. They could get their heads out of the way, so to speak, by stooping down, but are not sure if they could do so by stepping aside. Each spectator, however, could see the shadows of the bystanders as well as his own projected on to the appearance. They did not observe any rays spreading from it. The central disc, they think, is a reflected image of the sun, and the enclosing ring is a rainbow. The ring was in thickness about one-fourth of the diameter of the disc, and distant from it by about the same extent; but the recollection of one informant was that the ring touched the disc, without any intervening space. The shadow of a head, when thrown upon it, covered about one-eighth of the whole diameter of the meteor. The rainbow ring was not quite complete in its lower part, but they attribute this to the interposition of the edge of the precipice. They see no reason why the appearance should not be visible at night when the moon is brilliant and appositely placed. They profess themselves to have been a good deal surprised, but not startled, by the spectacle. They would consider it remarkable rather than astonishing, and are disposed to call it a very impressive phenomenon.

It is to be regretted that Mr. Baber failed to see the 'Glory,' and that we in consequence miss his own description of it. There seems a slight inadvertence in the statement that the head could be got out of the way by stooping; for, as long as the Glory' remained a circle, the shadow of the head must have occupied its centre. Stepping aside would simply displace the bow, but not abolish the shadow.

Thus, starting from the first faint circle seen drawn through the thick darkness at Alp Lusgen, we have steadily followed and developed our phenomenon, and ended by rendering the 'Glory of Buddha' a captive of the laboratory. The result might be taken as typical of larger things.

JOHN TYNDALL.

NOTE.

At the last moment, by desire of the Editor, I append here two diagrams reduced from their originals in the excellent optical atlas of Engel and Shellbach, Berlin. To render essential points clear, unessential details are omitted from the diagrams. We will first deal with the primary bow. M, fig. 1, is the centre of a spherical raindrop, with the circle god for its boundary. The line F I,

passing through м, is what I have called the central ray, which, on striking the back of the drop, is in part reflected back along its own course. The lines above FI represent parallel rays, each of which on entering the drop is refracted, on reaching its back is reflected, and finally emerges from the drop below the central ray. The angle enclosed between the incident and emergent rays is found by producing these rays until they intersect behind the drop. This angle increases in size up to a certain point, where it attains a maximum value, diminishing afterwards. In the diagram the particular ray, ab, which suffers the maximum deviation, is represented by a line thicker than the others. Its course can be followed through the drop to c, thence to d, where it emerges and passes on to the eye.

A mere inspection of the figure will show that a considerable body of rays in the neighbourhood of a b converge almost exactly on the point c. They are here reflected, and quit the drop nearly parallel to each other. In reality the rays cut each other at angles of infinitesimal magnitude, and form by their intersection a caustic or line of intense illumination. By the waves thus kept together the light of the rainbow is carried to the eye.

Within the drop it will be noticed that through the intersection of the rays the light is heaped up along the curved line c н, which is also a caustic.

In fig. 2 we deal with the secondary bow, in which the ray of minimum deviation and its neighbours, after two reflections within the drop, carry the light of the bow to the eye. Here the rays strike the lower hemisphere of the drop and are refracted on entering it. The ray of minimum deviation enters the drop at b, reaches the back of the drop at c, is reflected from that point to d, from which it is again reflected to e. Here it is refracted, and, supported by its neighbour rays, passes on to the eye. Before reaching the eye, it crosses the incident rays, and to this crossing, as stated in the text, the inversion of the colours is due.

The beautiful curved line shown within the drop in fig. 2, is a caustic produced, as before, by the intersection of the rays. The darkish area к нo р-darkness here representing excess of light-is bounded by four caustics. Such would be the aspect presented by a drop of water, could we see it in the act of producing the secondary bow.'

13

13 With a white basin or cup, and a candle held opposite the interior concave surface, caustics by reflection can be produced at will. A little practice is here required. in finding the best position for the light,

A