Rainbow. An effect on drops of rain, from the different refrangibility of the rays of light which produce different colours. The inner bow is produced by a refraction at the entrance of the drop, a reflection from its back, and a refraction at its exit, or by two refractions and one reflection. The outer bow by two refractions and two reflec-tions within the drop ; and, as much of the bight is dispersed by the two reflections the outer bow is fainter than the inner bow. As all the drops in a shower are affected at the same time, so all the colours of light are visible anywhere to a spectator whose is to the sun, in circles which are from 55 to 52 degrees, and from 42 1/2 to 40 degrees from the eye; but in the two bows the colours are reversed. The two bows may be imitated by small glass globes of water placed within the above angles from the eye, or by throwing up water with an engine. A line passing from the sun through the eye of the spectator goes to the centre of the bow, so that the height of the how is inversely as the height of the sun ; and, if the sun is more than 42 or 54 degrees high, there ran be no bow. Of course, as it is a mere optical effect, depending on the position of the eye, no two persons can see the same bow. The moon, or any strong light, will make a coloured bow as well as the sun.

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