58 



OPTICS. 



escape the inundations of the Nile. In the 

 morning and evening: objects are seen in 

 their natural form and position, but w hen 

 the surface of the sandy ground is 

 heated by the sun, the land seems ter- 

 minated at a particular distance by a 

 general inundation ; the villages which 

 are beyond it appear like so many 



islands in a great lake,-and between each 

 village an inverted image of it is seen. 



Our limits will not permit us to give 

 any farther examples of these curious 

 phenomena. We shall, therefore, at- 

 tempt to give a popular explanation of 

 their cause. 



Fig. 54 



Let S H (fig. 54.) be a ship in the 

 horizon, and visible to the eye at E, by 

 rays S E, H E proceeding in straight 

 lines to E, through a tract of the atmo- 

 sphere in its usual state.* If we sup- 

 pose, what is known to be sometimes 

 the case, that the refractive power of 

 the atmosphere, or air, above the line 

 S a E varies, so as to be less at c than 

 at a, then rays S d, H c proceeding 

 upwards from the ship, and that never 

 could in the ordinary state of the air 

 reach the eye at E, will be refracted 

 into curve lines H c, S d ; and if the 

 variation of refractive power is such, 

 that these last rays cross each other at x, 

 then the ray 8 d, in place of being the up- 

 permost, will now be the undermost, and, 

 consequently, will enter the eye as if it 

 came from the lower end of the object. 



If we now draw lines E s, E h tan- 

 gents to these curve lines at E, these 

 lines will be the direction in which the 

 ship will be seen by the rays H c, S d, 

 and the observer at E will see an in- 

 verted image s h of the ship S H consi- 

 derably elevated above the horizon. 

 The refractive power of the air still con- 

 tinuing to diminish, other rays, H m, 



f * We do not here consider that rays of light mov- 

 ing through tht> atmosphere nrc bent info curve lines 

 When the atmosphere is iu its usual strife; 1> 

 the effect is very small, and the consideration of it 

 would tend only to make the present explanation 

 more complex. ^ 



II n, that never could reach the eye at 

 E in the ordinary state of the atmo- 

 sphere, may likewise be bent into 

 curves which will not cross each other 

 before they reach the eye at E. In this 

 case, the tangent E s' to the upper curve 

 S n E will be uppermost, and the -tangent 

 E h' to the lower curve S m E lower- 

 most, so that the observer at E will see 

 an erect image s' h' of the ship above 

 the inverted image. It is possible that 

 a third, and even a fourth image may 

 be seen. 



If the variation of refractive power 

 takes place only in the tract of air 

 througl i which the rays H c, S d pass, 

 then there may only be an inverted 

 image ; and if it takes place only in the 

 tract through which S m, S n pass, 

 there may only be an erect image. It 

 is also obvious, that if the variation of 

 refractive power commences at the line 

 joining the eye and the horizon, the ordi- 

 nary image S H will not be seen ; and, 

 in like manner, it is clear that the in- 

 verted and erect images ,9 h, s' h' may be 

 seen even if the real ship S II is below 

 the visible horizon. 



In the case of Dover castle, the rays 

 from the top and bottom of the castle 

 passed above the hill in curve lines, 

 and the top of the hill was seen by the 

 observer at Ramsgate, by means of a 

 curved ray which reached the eye 



