Scientific American 8, 10.1.1863
We recently received a letter from a coppespondent in relation to primary and secondary rainbows, an we have since found the following facts upon this subject in the Journal of Popular Sciences: -
"There are frequently two rainbows seen, primary and secondary; the former is by far the brightest one, being formed by the rays of light falling on the upper part of the rain drops; for a ray of light, entering the upper part of a drop of water, will by refraction be thrown upon the inner part of the spherical surface of that drop, where, undergoing a second refraction it will be sent toward the eye of the spectator. Since the rays which fall upon the primary bow come to the eye after two refractions and one reflection, and the colors of this bow, reckoning outward, are violet, indigo, blue, green, yellow, orange and red. The secondary bow is formed by the ras of light falling on the lower part of the drops of rain. These rays, like the former, undergo two refractions, namely, when entering the drops of rain and when emerging from them in passing to the eye, but they suffer two or more reflections in the interior surface of the drops, hence the colors of their rays are not so strong or so well defined as those in the primary bow and appear in an incerted order."