Halo (optics)

“ Still it brightened, and now assumed the form of a ring of purest radiance, shining above his forehead; if a circle of pearls could be fire, they would appear thus. It was visible for several minutes; and whether it then vanished, or whether the eyes of the onlookers were unable any longer to discern it, was doubtful. Perhaps it was a thing which only persons of good will and pure heats could have seen at all.
They went out in silence; but the meaning of the halo sank deep into the lovers’ souls, and its light guided their life. ”

“ The halo diminishes in brightness from the centre outwards, and is probably due to the diffraction of light. Under favourable conditions four concentric rings may be seen round the shadow of the observer’s head, the outermost, which seldom appears, having an angular radius of 40°. ”

“ And if the Hail be a little flatted, as it often is, the Light transmitted may grow so strong at a little less distance than that of 26 Degrees, as to form a Halo about the Sun or Moon; which Halo, as often as the Hail-stones are duly figured may be colour'd, and then it must be red within by the least refrangible Rays, and blue without by the most refrangible ones, especially if the Hail-stones have opake Globules of Snow in their center to intercept the Light within the Halo (as Hugenius has observ'd) and make the inside thereof more distinctly defined than it would otherwise be. ”

“ In such a telescope of any considerable aperture the image of a bright star at focus is surrounded by a halo of bluish or violet-coloured light,—a defect which is unavoidable in an object-glass composed of a crown and flint lens on account of the irrationality of their spectra (Light, vol. xiv. p. 592) . There seems to be no doubt that different eyes are differently impressed by rays of different wave-length. ”

“ I just brought my cloud stuff this time."
"Well," said the Forecaster, "suppose you put one of those in here as an example of cirro-stratus. There couldn't be a halo without it. All the upper clouds are made of ice crystals and it is the refraction of the sunlight through these ice crystals that forms most halos. By the way, boys, don't confuse a halo with a corona. They're quite easy to tell apart, because a halo, unless it is one of the unusual white ones, always has red as the inside [Pg 153] color and a corona always has the red on the outside." ”

“ Unlike the halo, the ray phenomena are familiar and involve no new principle, but the idea of rays around a black spot is new to me, and quite as important as the halo in its application to visual work by telescope or microscope. As all know, the rays on a star are produced by irregular refraction in the eye, originating in what are known as the stellate figures. The figures seem to be construction lines, as it were, in the crystalline lens, and develop during its growth. They are permanent in form, when adult years are reached, and may be ”

“ There are those who say it never can be reached, and that the halo which rests upon its slopes is no earthly light, but rather, as it were, a promise and a beacon—a glow reflected whence we know not, and lying on this alien earth as the sun’s light lies on the dead bosom of the moon. ”

“ Reflection from an excess of horizontal prisms gives rise to a vertical circle passing through the sun.
The parhelia (p) were explained by Mariotte as due to refraction through a pair of alternate faces of a vertical prism. When the sun is near the horizon the rays fall upon the principal section of the prisms; the minimum deviation for such rays is 22°, and consequently the parhelia are not only on the inner halo, but also on the parhelic circle. As the sun rises, the rays enter the prisms more and more obliquely, and the angle of minimum deviation increases ”

“ A halo may form round the sun as well as the moon ... but a halo is more frequently noticed round the moon for the reason that we are too much dazzled by the sun’s light to distinguish faint colours surrounding its disc, and to see them it is necessary to look through smoked glass, or view the sun by reflection from the surface of still water, by which its brilliancy is very much reduced.”... ”

“ Now it is not improbable that the halo may contribute directly to
this discussion. We can evidently attack
240
the biotite with a known number of alpha rays and determine how
many are required to produce a certain intensity of darkening,
corresponding to that of a halo with a nucleus of measurable
dimensions. On certain assumptions, which are correct within
defined limits, we can calculate, as I have done above, the
number of rays concerned in forming the halo. In doing so we
assume some value for the age of the halo. Let us take the
maximum radioactive value. ”

“ It is clear, therefore, that earlier observers had witnessed the same phenomenon, although they were unable to offer any explanation of it. It seems, however, no unreasonable conclusion to come to, that these projections or red clouds, as well as the halo with its pencils of light before spoken of, are something without the proper solar photosphere, but not forming, as this does, one connected mass of light. ”

“ In physical science, a halo is a luminous circle, surrounding the sun or moon, with various auxiliary phenomena, and formed by the reflection and refraction of light by ice-crystals suspended in the atmosphere. ”

“ It had a dense, black spot in its very center, usually well rayed; then, a light zone limited by an intense black ring, which in turn produced its own complete halo. This form is shown in Fig. 5.
This mote observation is by no means easy. I have often waited fifteen minutes for a mote of this type to appear, and only once have I kept one in sight for any length of time. It then remained in the center of vision for at least twenty minutes. Usually, they float past the center of the vision and give one only a brief view. The size of pin hole used is 1/50 inch. ”

“ Hence a dense halo close to the moon is universally and justly regarded as an indication of coming rain. It has been stated as an approximation, that the globules which occasion the appearance of colored circles, vary from the 5000th to the 50,000th part of an inch in diameter. Though seldom apparent around the sun in our climate, yet it is only necessary to remove that glare of light which makes delicate colors appear white, to perceive segments of beautifully tinted halos on most days when light fleecy clouds are present. ”

“ The halo, the deep water—streaming through the rent cloud, glowing in the coal, quivering in the lightning, flashing in the topaz and the ruby, veiled behind the pure alabaster, mellowed and clouding itself in the pearl-light contrasted with shadow, shading off and copying itself in the double rainbow like voice and echo—light seen within light—light from every source and in all its shapes illuminates, irradiates, gives glory to the Commedia.... ”

“ It was about 60° of a circle of a smaller periphery than the halo, and well defined. The part nearest the sun was found to be 48° 17 ́ distant from that luminary, so that it did not come in contact with the halo, but the points of their nearest approach were 25° 37 ́ apart. The halo was indistinct, except in the vicinity of the mock suns, where it was well defined. Time of making observations half past three o'clock, P.M. Thermometer at the same time 5°, Barometer 28.88, atmosphere hazy, thin light clouds about sun, fresh N.W. wind. ”

“ The halo he wears illumines the whole world. I cannot endure its radiance even a thousand miles away, so I must hurry and get out of the way!” And with that he stepped into a cloud and disappeared. The Master of the Heavens then told the mandarin what had happened, and added: “Fortunately you did not see the God of War face to face! Whoever does not possess the greatest virtue and the greatest wisdom, would be melted by the red [72] glow of his halo.” So saying he gave him a pill of the elixir of life to eat, and his blind eye gradually regained its sight. ”

“ A popular London divine, having noticed a bright ring round Alcyone on a photograph of the group, took that halo, which every photographer would at once recognize as a mere photographic defect, as a confirmation of this baseless fancy. Foolishness of this kind has nothing to support it in science or religion; it is an offence against both. We have no reason to regard the Pleiades as the centre of the universe, or as containing the attracting mass which draws our sun forward in its vast mysterious orbit. ”

“ Extending all round the head of the pin at a distance of about 7′ of arc (one inch at a distance of 500 inches) is an intensified zone in which the color of the background appears stronger; and outside of that a reduction zone, or ring, or secondary image, in which the intensity of the background is reduced by the addition of some of the color of the spot observed.
In order to find the cause of this halo, many tests were made, of which the first was upon the size of the central spot, It was found that the distance from the edge of the spot to the secondary image is constant ”

“ The inventors claim that mirrors so prepared are able to compete successfully with lenses and totally reflecting prisms—in fact, it was maintained that the silvered glass parabolic mirror possessed the advantages of greater reflecting power and enhanced accuracy, with less divergence of the beam of light.
Owing to the perfection of the lenses and prisms system of lighthouse optics, the introduction of arc lights in conjunction with parabolic mirrors was received with considerable hesitation. ”

“ The moon had escaped from the cloud-pack, and was radiant behind a fine veil which glistened to her rays, and which was broidered with a lustrous halo, very large indeed, the largest halo Siegmund had ever seen. When the little lane turned full towards the moon, it seemed as if Siegmund and Helena would walk through a large Moorish arch of horse-shoe shape, the enormous white halo opening in front of them. They walked on, keeping their faces to the moon, smiling with wonder and a little rapture, until once more the little lane curved wilfully, and they were walking north. ”

“ This beautiful star-swarm, visible to the naked eye as a glowing patch in the Milky Way, is indicated in Chart X by a double cluster of dots above the star Eta (η) . Seen with a powerful opera-glass, or better with a small telescope, it is an object that one can never cease to admire and wonder at. It is so bright that the unassisted eye sees it as soon as it is directed toward that part of the sky. It seems to throw a halo over the surrounding sky, as if at that point the galaxy had been tied into a gleaming knot. It is popularly called the “Sword Hand of Perseus.” ”

“ The eye-piece enables the observer to see this image to the best advantage.
The functions of the objective as well as those of the eye-piece may, to a certain extent, each be performed by a single lens. Galileo and his contemporaries made their telescopes in this way, because they knew of no way in which two lenses could be made to do better than one. But every one who has studied optics knows that white light passing through a single lens is not all brought to the same focus, but that the blue light will come to a focus nearer the objective than the red light. ”

“ As a halo is never seen except when the sky is hazy, it indicates that moisture is accumulating in the atmosphere which will form clouds, and usually result in a storm. But the popular notion that the number of bright stars visible within the circle indicates the number of days before the storm will occur, is without any foundation whatever, and the belief is almost too absurd to be refuted. In whatever part of the sky a lunar halo is seen, one or more bright stars are always sure to be noticed inside the luminous ring, and the number visible depends entirely upon the position of the moon. ”

“ The difference in radius should be noticeable.
If, on the other hand, actinium
was always associated with uranium and radium, then its effects
would be submerged in those of the much more potent effects of
the uranium series of elements.
It will have occurred to you already that if the radioactive
origin of the halo is assured the shape of a halo is not really
circular, but spherical. This is so. There is no such thing as a
disc-shaped halo. The halo is a spherical volume containing the
radioactive nucleus at its centre. ”

“ Those not spiritually evolved have a halo of a very different color! Artists of old painted a golden halo around the pictures of saints because the artists actually saw the halo, otherwise he would not have painted it. The halo is merely a very small part of the human aura, but is more easily seen because it is usually much brighter. ”

“ SUCCESSIVE STAGES OF THE UPPER AND LOWER TANGENT ARCS OF THE 22° HALO
When the sun is high they unite to form the “circumscribed halo.” (Altitude of sun shown in the center of each figure.)
The ice crystals that produce halos consist of hexagonal plates or columns, occasionally including complications of structure, such as pyramidal bases, combinations of plates and columns, etc. These have the well-known effect of prisms in refracting and dispersing light that passes through them. ”

“ The defects of the Galilean and Kepler telescopes are due to the chromatic and spherical aberration of the simple lenses of which they are composed. The substitution of a positive or negative eye-piece for the simple convex or concave eye-lens, and of an achromatic object-glass for the simple object-lens, transforms these early forms into the modern achromatic telescope. The Galilean telescope with a concave eye-lens instead of an eye-piece still survives as the modern opera-glass, on account of its shorter length, but the object-glass and eye-lens are achromatic combinations. ”