Page:Optics.djvu/161

 walking from one, for instance, presents successively the appearances represented in Fig. 205.

The French sailors give the name of Mirage to this phænomenon; the Italian peasants attribute it to the Fata Morgana. According to Dr. Young, it is known in this country by the name of Looming.

Sometimes a distant object appears suspended in the air, without any inverted image. In this case the image does really exist, but it is so extremely thin that it becomes imperceptible.

179. Effects such as we have been describing, may be produced artificially, by introducing some sulphuric acid through a funnel underneath pure water in a glass vessel. The acid being the heavier of the two liquids, raises the water above itself, and remains in part undiluted, but as it has a very strong affinity for water, the fluids combine where they are in contact, and thus there is formed between the pure acid and the pure water, a mixture passing insensibly from the one to the other, and decreasing continuously in refractive power. If now an object be looked at through the acid, it will be seen directly in its natural place, but there will also be an inverted image above it, produced by rays emitted upwards into the mixture, and refracted downwards again to the eye, (see Fig. 206.)

An appearance more exactly like the mirage, may be observed by looking at an object along a heated surface, such as that of a stove, or a hot poker. The air in its vicinity becomes very much rarified, and if the eye be moved about a little, it will be observed, that when a distant object is nearly in a line with the edge of the heated substance, it is seen double.

180. This phænomenon is caused by the drops of rain refracting and reflecting the rays of the Sun.

The usual appearance is one bow, consisting of concentric stripes, coloured like the prismatic spectrum, the violet being on the inside: above that is often seen another which is of course wider, but otherwise differs from the former only in having the colours in the inverse order, and rather less distinct. Sometimes a third, and even a fourth bow may be seen, but they are always extremely faint. The manner of this formation is shewn in Fig. 207, where