RAINBOW. See OPTICS.
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 Alverstoke, 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 solution 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; and there are scarcely 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. Marlotte, in his fourth Essai de Physique, says, these bows are formed in mists, as the others are in showers; and adds, that he has seen several both after sunrising 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 tenuity of the little vesiculae of the vapour, which being only little watery pellicles bloated 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. Rohault mentions coloured rainbows on the grass; 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. 1. p. 294. we 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.