EXPLANATION OF SPECTRE SHIPS. 



149 



will see the top-mast of the ship S, directly 

 through the plate of glass B D. Fix a convex 

 lens a of short focus upon the plate of glass B D, 

 and a little above a straight line S E joining the 

 ship and the eye; and immediately above the 

 convex lens a place a concave one b. The eye 

 Fig. 35. 



will now see, through the convex lens a, an in- 

 verted image of the ship at S', and through the 

 concave lens b, an erect image of the ship at S", 

 representing in a general way the phenomena 

 shown in Fig. 33. But it will be asked, where 

 are the lenses in nature to produce these effects r 

 This question is easily answered. If we take a 

 tin tube with glass plates at each end, and fill it 

 with water, and if we cool it on the outside with 

 ice, it will act like a concave lens when the cool- 

 ing effect has reached the axis ; and, on the other 

 hand, if we heat the same tube filled with water, 

 on the outside, it will act as a convex glass. In 

 the first case the density of the water diminishes 

 towards the centre, and in the second it increases 



