RAINBOW, a circular arch of variously coloured light which is visible in the heavens when the sun or moon is shining, and when, at the same time, a shower of rain is falling on the opposite side of the spectator. When the rain is abundant, a second bow is commonly area on the exterior, and concentric with the first ; their common centre being in a line drawn from the luminary through the eye of the spectator and produced towards the opposite part of the heavens. Both bows consist of concentric bands of the different prismatic, colours arranged as they appear in the solar spectrum, but the order in which they are disposed in the first bow is inverted in the second. The lower edge of the interior bow is violet and the upper edge is red ; on the contrary, the lower edge of the exterior bow Is red and the upper edge is violet.
The rainbow is a phenomenon which appears at all times to have been understood to depend upon the light of the sun or moon and the drops of falling rain; but the first complete explanation of the circum stances connected with it is due to Newton (` Optics,' book i., p. 2, prop. 9). In the beginning'of the 16th century no better notion was entertained of the cause of the phenomenon than that the Interior bow was a distorted reflection of the sun's imago from the surface of a cloud, and that the exterior bow was a reflected image of the first. But the reflection of light is not capable of producing different colours, and it is said that Fleischer of Breslau (1571) was the first who enter tained the idea that the particles of light from the sun entered into the drops of rain. His opinion was that a ray of light suffered one refraction on entering and another on leaving a drop ; and that it entered the eye of the spectator after reflection from the surface of a second drop. It appears that Kepler, in a letter to Harriet (1606), suggested that the particles of light, in a ray which is a tangent to some part of the surface of a drop of rain, might enter the drop by refraction, and that this ray, being subsequently reflected at the interior surface of the latter, might enter the eye of the spectator after being again refracted on leaving the drop. The hypothesis is worthy
of Kepler's sagacity ; and, as far as it goes, it differs from the fact only in the manner in which the incident ray is supposed to fall on the drop. Newton ascribes the first idea of the true explanation to Antonio do Dominic, bishop of Spalatro, whose work, 'De Mullis visas; was published in 1611, but Is said to have been composed In 1590: the work however appears to have been so obscurely written, and to betray so much ignorance of the laws of optics, that it is doubtful whether or not the author had any more than a vague conception of the cause of the colours. (See Montucla, a Histoire des Math.; tom.
Descartes is certainly the first who has distinctly explained the causes by which the two bows are produced, and he states (' Meteors; cap. viii.) that he detected those causes on observing the phenomena presented by a glass globe filled with water, which he placed in various positions with respect to the sun. He shows that the interior or primary bow is produced by rays from the sun falling upon the drops of rain near their upper surfaces, where, being refracted, they pass to the side ofthe drop which is farthest from the sun and spectator ; from thence they aro reflected towards the lower surface, and, on quitting the drop, they suffer a second refraction. He shows also that the exterior or secondary bow is produced by rays from the sun falling upon the drops of rain near their lower surfaces, where, being refracted, they pass, as before, to the farther side of the drop ; from thence they Now It Is not sufficient that the pencils of light which are incident on the drops of rain should be so refracted and reflected ; it is moreover necessary that each pencil on emerging from the drop should consist of parallel rays of light, that, when It enters the eye of the spectator, it may produce in the mind the perception of brightness ; and Descartes determined by computation the positions of the incident and emergent rays so that this effect may be produced.