586 Proceedings of the Royal Society 
Evidence of this character would not he received in support of 
the most common-place assertion ; hut the phenomenon in question, 
far from being common-place, may, rather, he described as miraculous. 
Dover Castle is situate on the south side of a hill ; Ramsgate is far 
away on the north side ; and the tops of the turrets of the Castle 
are usually seen from Ramsgate just above the hill. But, on this 
occasion, the Castle was seen as if it had been brought to the north 
side, and, apparently, in its natural position with regard to the 
turrets. Vince tells us that the light from the hill undoubtedly 
reached the eye, hut that it was so overpowered by the greater light 
from the Castle as to he unperceived ; he places before us this draw- 
ing to represent the Castle as he saw it, and proceeds to explain 
how this vision may have been brought about. 
For this purpose he constructs an abstract figure in which AB 
represents the Castle, T the top of the hill, E the place of the 
spectator ; and he draws two curved lines AyvE, ExivE to represent 
the paths of the light from the top and bottom of the Castle 
to the eye. He draws also the curve TxzE as the path of the light 
emanating from the top of the hill, crossing ExwE at the point x, 
and having its part xzE between the two first mentioned lines. He 
says, “ Then it is manifest that such a disposition of rays wdll 
produce the observed appearance ..... The phenomenon cannot 
otherwise be accounted for.” 
According to his explanation, AyvE and ExwE are the paths 
of the light in the usual state of the atmosphere, and the pheno- 
menon in question is produced by the rapid curvature of the part 
xz of the ray ExzE ; this rapid curvature being due to a variation 
in the density of the air between yvE and xwE. 
Supposing, merely for the sake of fixing our ideas, the height of 
the tower BA to be 100 feet; the separation of the curves at 
x and y may be say 60 feet, within which distance this extra- 
ordinary disposition of the air must exist, and not beyond on either 
side, for otherwise the paths A?/E, ExE would be deranged. In 
