EXPLANATION OP SPECTRE SHIPS. 



219 



positions. The reason why the image of the castle and 

 the hill appeared erect was that the rays from the top and 

 bottom of the castle had not crossed before they reached 

 Eamsgate; but as they met at Kamsgate, an eye at a 

 greater distance from the castle, and in the path of the 

 rays, would have seen the image inverted. This will be 

 better understood from the annexed diagram, which re- 

 presents the actual progress of the rays, from a ship S P, 

 concealed from the observer at E by the convexity of the 



Fig. 36. 



earth P Q E. A ray proceeding from the keel of the 

 ship P is refracted into the curve line P c x c E, and a 

 ray proceeding from the topmast S is refracted in the 

 direction S d x d E, the two rays crossing at aj, and pro- 

 ceeding to the eye E with the ray from the keel F 

 uppermost ; hence the ship must appear inverted as at s p. 

 Now if the eye E of the observer had been placed nearer 

 the ship as at x, before the rays crossed, as was the case 

 at Eamsgate, it would have seen an erect image of the 

 ship raised a little above the real ship S P. Rays S m, 

 S n, proceeding higher up in the air, are refracted in the 

 directions S in m E, S n n E, but do not cross before they 

 reach the eye, and therefore they afford the erect image 

 of the ship shown at s' p. 



