120 



THE DESERT 



Ships up- 

 tide down. 



Wherein the 

 illusion. 



coming from the ship on the water describe an 

 obtuse angle or curve in reaching the eye. The 

 rays from the bottom of the ship, lying in a 

 dense part of the air-lens, are more acutely 

 bent than those from the masts, and hence they 

 go to the top of the photographic plate or your 

 field of vision, whereas the rays from the ship's 

 masts, being in a thinner atmosphere, are less 

 violently bent, and thus go to the bottom of your 

 field of vision. The result is the ship high in 

 air above the horizon-line and upside down. 



The illusion or deception consists in this : 

 We usually see things in flat trajectory, so to 

 speak. Light comes to us in comparatively 

 straight rays. The mind, therefore, has formu- 

 lated a law that we see only by straight rays. 

 In the case of mirage the light comes to us on 

 curved, bent, or angular rays. The eyes recog- 

 nize this, but the mind refuses to believe it and 

 hence is deceived. We think we see the ship 

 in the air by the straight ray, but in reality we 

 see the ship on the water by the bent ray. It 

 is thus that ships are often seen when far below 

 the horizon-line, and that islands in the sea be- 

 low the ocean's rim, and so far away as a hun- 

 dred miles, are seen looming in the air. " Loom- 

 ing " is the word that describes the excessive 



