Rainbow. See Optics, Part II. Sect. i. § 1.

In the Philosophical Transactions for 1793, we have the following account of two rainbows seen by the Rev. Mr Sturges.

"On the evening of the 9th of July 1792, between seven and eight o'clock, at Alverstone, near Gosport, on the sea-coast of Hampshire, there came up, in the south-east, a cloud with a thunder-shower; while the sun shone bright, low in the horizon to the north-west.

"In this shower two primary rainbows appeared, AB and AC, not concentric, but touching each other at A, in the south part of the horizon; with a secondary bow to each, DE and DF (the last very faint, but discernible), which touched likewise at D. Both the primary were very vivid for a considerable time, and at different times nearly equally so; but the bow AB was most permanent, was a larger segment of a circle, and at last, after the other had vanished, became almost a semicircle; the sun being near setting. It was a perfect calm, and the sea was as smooth as glass.

"If I might venture to offer a foliation of this appearance, it would be as follows. I consider the bow AB as the true one, produced by the sun itself; and the other, AC, as produced by the reflection of the sun from the sea, which, in its perfectly smooth state, acted as a speculum. The direction of the sea, between the Isle of Wight and the land, was to the north-west in a line with the sun, as it was then situated. The image reflected from the water, having its rays issuing from a point lower than the real sun, and in a line coming from beneath the horizon, would consequently form a bow higher than the true one AB. And the shores, by which that narrow part of the sea is bounded, would before the sun's actual setting intercept its rays from the surface of the water, and cause the bow AC, which I suppose to be produced by the reflection, to disappear before the other."

The marine or sea bow is a phenomenon which may be frequently observed in a much agitated sea, and is occasioned by the wind sweeping part of the waves, and carrying them aloft; which when they fall down are refracted by the sun's rays, which paint the colours of the bow just as in a common shower. These bows are often seen when a vessel is sailing with considerable force, and dashing the waves around her, which are raised partly by the action of the ship and partly by the force of the wind, and, falling down, they form a rainbow; and they are also often occasioned by the dashing of the waves against the rocks on shore.

In the Philosophical Transactions, it is observed by F. Bourzes, that the colours of the marine rainbow are less lively, less distinct, and of shorter continuance, than those of the common bow; that there are scarce above two colours distinguishable, a dark yellow on the side next the sun, and a pale green on the opposite side. But they are more numerous, there being sometimes 20 or 30 seen together.

To this class of bows may be referred a kind of white or colourless rainbows, which Mentzelius and others affirm to have seen at noon-day. M. Marbotte, in his fourth Élémens de Physique, says, these bows are formed in mists, as the others are in showers; and adds, that he has seen several both after sun-rising and in the night. The want of colours he attributes to the smallness of the vapours which compose the mist; but perhaps it is rather from the exceeding tenacity of the little vesicule of the vapour, which being only little watery pellicles floated with air, the rays of light undergo but little refraction in passing out of air into them; too little to separate the differently coloured rays, &c. Hence the rays are reflected from them, compounded as they came, that is, white. Robaut mentions * coloured rainbows on * Trois études sur les grès; formed by the refractions of the sun's rays in the morning dew. Rainbows have been also produced by the reflection of the sun from a river; and in the Philosophical Transactions, Vol. I., p. 294, we have Rainbow. Have an account of a rainbow, which must have been formed by the exhalations from the city of London, when the sun had been set 20 minutes, and consequently the centre of the bow was above the horizon. The colours were the same as in the common rainbow, but fainter.

It has often been made a subject of inquiry among the curious how there came to be no rainbow before the flood, which is thought by some to have been the case from its being made a sign of the covenant which the Deity was pleased to make with man after that event. Mr Whitehurst, in his Inquiry into the Original State and Formation of the Earth, p. 173, &c. endeavours to establish it as a matter of great probability at least, that the antediluvian atmosphere was so uniformly temperate as never to be subject to storms, tempests, or rain, and of course it could never exhibit a rainbow. For our own part, we cannot see how the earth at that period could do without rain any more than at present; and it appears to us from Scripture equally probable that the rainbow was seen before the flood as after it. It was then, however, made a token of a certain covenant; and it would unquestionably do equally well for that purpose if it had existed before as if it had not.

Lunar Rainbow. The moon sometimes also exhibits the phenomenon of an iris or rainbow by the refraction of her rays in drops of rain in the night-time. This phenomenon is very rare. In the Philosophical Transactions for 1783, however, we have an account of three seen in one year, and all in the same place, communicated in two letters by Marmaduke Tunstall, Esq. The first was seen 27th February 1782, at Greta Bridge, Yorkshire, between seven and eight at night, and appeared "in tolerably distinct colours, similar to a solar one, but more faint: the orange colour seemed to predominate. It happened at full moon; at which time alone they are said to have been always seen. Though Aristotle is said to have observed two, and some others have been seen by Stellius, &c. I can only find two described with any accuracy; viz. one by Plot, in his History of Oxfordshire, seen by him in 1675, though without colours; the other seen by a Derbyshire gentleman at Glapwell, near Chesterfield, described by Thoresby, and inserted in No. 331. of the Philosophical Transactions: this was about Christmas, 1710, and said to have had all the colours of the Iris Solaris. The night was windy; and though there was then a drizzling rain and dark cloud, in which the rainbow was reflected, it proved afterwards a light frost."

Two others were afterwards seen by Mr Tunstall; one on July the 30th, about 11 o'clock, which lasted about a quarter of an hour, without colours. The other, which appeared on Friday October 18, was "perhaps the most extraordinary one of the kind ever seen. It was first visible about nine o'clock, and continued, though with very different degrees of brilliancy, till past two. At first, though a strongly marked bow, it was without colours; but afterwards they were very conspicuous and vivid in the same form as in the solar, though fainter; the red, green, and purple, were most distinguishable. About twelve it was the most splendid in appearance; its arc was considerably a smaller segment of a circle than a solar; its south-east limb first began to fail, and a considerable time before its final extinction: the wind was very high, nearly due west, most part of the time, accompanied with a drizzling rain. It is a singular circumstance, that three of these phenomena should have been seen in so short a time in one place, as they have been esteemed ever since the time of Aristotle, who is said to have been the first observer of them, and saw only two in 50 years, and since by Plot and Thoresby, almost the only two English authors who have spoke of them, to be exceeding rare. They seem evidently to be occasioned by a refraction in a cloud or turbid atmosphere, and in general are indications of stormy and rainy weather: so bad a season as the late summer having, I believe, seldom occurred in England. Thoresby, indeed, says, the one he observed was succeeded by several days of fine serene weather. One particular, rather singular, in the second, viz. of July the 30th, was its being six days after the full of the moon; and the last, though of so long a duration, was three days before the full: that of the 27th of February was exactly at the full, which used to be judged the only time they could be seen, though in the Encyclopedia there is an account that Weidler observed one in 1719, in the first quarter of the moon, with faint colours, and in very calm weather. No lunar iris, I ever heard or read of, lasted near so long as that on the 18th instant, either with or without colours."

In the Gentleman's Magazine for August 1788 we have an account of a lunar rainbow by a correspondent who saw it. "On Sunday evening the 7th of August (says he), after two days, on both of which, particularly the former, there had been a great deal of rain, together with lightning and thunder, just as the clocks were striking nine, 23 hours after full moon, looking through my window, I was struck with the appearance of something in the sky, which seemed like a rainbow. Having never seen a rainbow by night, I thought it a very extraordinary phenomenon, and hastened to a place where there were no buildings to obstruct my view of the hemisphere: here I found that the phenomenon was no other than a lunar rainbow; the moon was truly 'walking in brightness,' brilliant as she could be; not a cloud was to be seen near her; and over-against her, toward the north-west, or perhaps rather more to the north, was a rainbow, a vast arch, perfect in all its parts, not interrupted or broken as rainbows frequently are, but unremittedly visible from one horizon to the other. In order to give some idea of its extent, it is necessary to say, that as I stood toward the western extremity of the parish of Stoke Newington, it seemed to take its rise from the well of Hampstead, and to the end, perhaps, in the river Lea, the eastern boundary of Tottenham; its colour was white, cloudy, or greyish, but a part of its western leg seemed to exhibit tints of a faint sickly green. I continued viewing it for some time, till it began to rain; and at length the rain increasing, and the sky growing more hazy, I returned home about a quarter or 20 minutes past nine, and in ten minutes came out again; but by that time all was over, the moon was darkened by clouds, and the rainbow of course vanished."

Marine Rainbow, or Sea-bow. See the article Rainbow.

Rainbow-Stone. See Moon-Stone.