OPTICAL PHENOMENA. 



543 



their hulls meeting, according as 

 the inverted image is above or be- 

 low the other. Dr. Wollaston has 

 shown that the production of these 

 images is owing to the refraction 

 of the rays through media of dif- 

 ferent densities. Looking along 

 a red-hot poker at a distant object, 

 two images of it were seen, one 

 erect and the other inverted, 

 arising from the change produced 

 by the heat in the density of the 

 air. A singular instance of lateral 

 mirage was noticed upon the Lake 

 of Geneva by MM. Jurine and 

 Soret in the year 1818. A bark 

 near Bellerive was seen approach- 

 ing to the city by the left bank 

 of the lake ; and at the same time 

 an image of the sails was observed 

 above the water, which, instead 

 of following the direction of the 

 bark, separated from it, and ap- 

 peared approaching by the right 

 bank the image moving from 

 east to west, and the bark from 

 north to south. When the image 

 separated from the vessel, it was 

 of the same dimensions as the 

 bark ; but it diminished as it re- 

 ceded from it, so as to be reduced to one-half when the appearance ceased. This was a 

 striking example of refraction, operating in a lateral as well as a vertical direction. 



6. Ignis fatuus. This wandering meteor, known to the vulgar as the Will-o'-the-Wisp, 

 has given rise to considerable speculation and controversy. Burying grounds, fields of 

 battle, low meadows, valleys, and marshes, are its ordinary haunts. By some eminent 

 naturalists, particularly Willoughby and Ray, it has been maintained to be only the 

 shining of a great number of the male glow-worms in England, and the pyraustae in Italy, 

 flying together an opinion to which Mr. Kirby, the entomologist, inclines. The lumi- 

 nosities observed in several cases may have been due to this cause, but the true meteor of 

 the marshes cannot thus be explained. We hare but a few authentic notices of its 

 appearance in this country of a recent date, probably owing to an extended system of 

 drainage, and the careful cultivation of the soil. The following instance is abridged from 

 'the Entomological Magazine : " Two travellers proceeding across the moors between 

 Hexham and Alston, were startled, about ten o'clock at night, by the sudden appearance 

 of a light, close to the road side, about the size of the hand, and of a well-defined oval 

 form. The place was very wet, and the peat-moss had been dug out, leaving what are 

 locally termed ' peat-pots,' which soon fill with water, nourishing a number of confervas, 

 and the various species of sphagnum, which are converted into peat. During the process 

 of decomposition, these places give out large quantities of gas. The light was about 

 three feet from the ground, hovering over the peat-pots, and it moved nearly parallel 



Refraction in the Polar Sea 



