THE MIRAGE. 1Q1 



observed at sea; hence the name (Mirage, sea vision), which the 

 French sailors gave it. 



We shall quote one or two remarkable cases of this optical illusion, 

 and then endeavour to give a philosophical explanation of its causes. 



At sea the mirage is usually noticed by the form, distinguished by 

 the term " suspension ;" the object is then represented as above the 

 water, painted, as it were, on the sky. None more striking of which 

 appears to be recorded, than that which was observed by Captain 

 Scoresby, on the 28th of January, 1820, in the Greenland Seas. The 

 sun was very brilliant, and his rays unusually ardent during the day. 

 In the evening a light breeze sprung up, and most of the ships navigating 

 at the distance of ten or fifteen miles (about eighteen or nineteen sail), 

 appeared then to undergo a great change of magnitude and form, and 

 when examined from the mast head with a telescope, exhibited some 

 very extraordinary appearances, differing in almost every point of the 

 compass. One ship had an inverted image above it, another two 

 distinct images in the air, a third was distorted by elongation, the masts 

 being nearly of twice the proper height, and others underwent contraction. 

 All the images of the ships were accompanied by a reflection of the ice, 

 in some places in two strata. 



The images of the mirage are usually vertical ; that is, presenting the 

 appearance of one object above another, like a ship above its shadow in the 

 water. Occasionally, but very rarely, they are lateral or horizontal, 

 (viz.) one or more images are represented upon the same plane with the 

 object. This form of the phenomena was seen by M. M. Jurine and 

 Soret,onthelakeof Geneva, in September, 1818. A bark, near Bellerive, 

 was seen approaching Geneva by the left bank of the lake, and at the 

 same time an image of the sails was seen above the water, which, instead 

 of following the direction of the bark, separated from it, and appeared to 

 appraoch Geneva by the right bank of the lake; the image moving from 

 east to west, while the vessel moved from north to south. When the 

 image separated from the object, it was of the same dimensions as the 

 bark, but it diminished as it receded, so that when the phenomena 

 ceased it was reduced one half! 



This remarkable class of optical illusions is thus accounted for : 

 when a ray of light strikes obliquely, a medium less refracting than 

 that in which it was previously moving, it is turned back into its original 

 medium, and a direction is given to it precisely similar to that which 

 would have been the result of a reflection taking place at the common 

 surface of the two mediums. Now the sand of the desert, or the surface 



