It will be understood, that since the eyes of different observers cannot be in precisely the same place at the same time, no two observers can see the _same_ rainbow; that is to say, the bow produced by one set of drops to the eye of one observer is produced by another set of drops to the eye of another observer. A rainbow can never be greater than a semicircle, unless the spectator is on elevated ground; for if it were greater than a semicircle the centre of the bow would be above the horizon, while the sun, which must be in a line drawn through that centre and the eye of the observer, would be below the horizon: but in such a case, the sun could not shine on the drops of rain, and consequently there could be no rainbow. When the rain cloud is of small extent only a portion of a bow is visible; when the cloud overspreads a large part of the sky a perfect bow appears. Sometimes the bow may be traced across a portion of blue sky, or it may appear to rest on the ground. In the former case, there are vapours in the air too thin to be seen, but sufficient to refract and reflect the rays of light; in the latter, the drops of rain, adhering to the grass and foliage, produce the same effect. A coloured bow, similar to that produced by rain, is sometimes seen in the spray of a fountain or of a water-fall, and also in mists that lie low upon the ground. In mountainous and stormy regions rainbows are often seen to great advantage. In the islands off the Irish coast the author of “Letters from the Irish Islands,” describes the rainbow of winter “as gradually advancing before the lowering clouds, sweeping with majestic stride across the troubled ocean, then, as it gained the beach, and seemed almost within one’s grasp, vanishing amid the storm of which it had been the lovely but treacherous forerunner. It is, I suppose, a consequence of our situation, and the close connexion between sea and mountain, that the rainbows here are so frequent and so peculiarly beautiful. Of an amazing breadth, and of colours vivid beyond description, I know not whether most to admire this aërial phenomenon, when suspended in the western sky, one end of the bow sinks behind the Island of Boffin, while at the distance of several leagues the other rests upon the misty hills of Ennis Turc; or when, at a later hour of the day, it has appeared stretched across the ample sides of Mulbrea, penetrating far into the deep blue waters that flow at its base. With feelings of grateful recollection, too, we may hail the repeated visits of this heavenly messenger, occasionally as often as five or six times in the course of the same day, in a country exposed to such astonishing, and, at times, almost incessant floods of rain.”