Prof. Everett on the Optics of Mirage. 171 



inverted image is visible when the erect image is hidden by the 

 convexity of the intervening water. 



Explanation. — A very rapid increase of temperature upwards in 

 a stratum of air overhead. The inverted images are formed by 

 rays incident from below upon this stratum at such an obliquity 

 that (as explained in V.) they cannot get through, but are com- 

 pelled to descend again, and thus undergo a kind of reflection. 

 The upward increase is supposed to be more sudden here than 

 in (1), and confined to a thinner stratum. 



3. Multiple images seen above the true position of the object. 

 Explanation. — Either several strata of rapid upward increase 



of temperature, each of them, as in (2), fulfilling the office of a 

 mirror, or a single such stratum of irregular shape, yielding re- 

 flections in different places. 



4. An appearance as of architectural columns, obelisks, spires, 

 or basaltic cliffs. Such appearances are said to be common in 

 the illusions of the Fata Morgana at the Straits of Messina; 

 and many instances are described, with illustrative plates, in 

 Scoresby's 'Greenland/ Such appearances are always due to 

 the vertical magnification of real objects. 



Explanation. — In the arrangements of III. and VI. an object 

 at a short distance in front of or behind a focus conjugate to 

 the position of ttie observer's eve, will be greatly magnified in 

 the vertical direction, its vertical diameter being seen under the 

 same angle as if the eye were at this conjugate focus. If between 

 the eye and the first conjugate focus, it will appear erect; if be- 

 tween the first and the second conjugate focus, inverted. T 

 believe that, when vertical magnification is exhibited with any 

 thing like regularity, an arrangement approximately resembling 

 that described in (VI.) prevails in the body of air which lies be- 

 tween the observer and the objects magnified. 



The same appearance is often seen on land (small bushes, for 

 example, being magnified into tall trees), and is to be similarly 

 explained. 



5. A false appearance of water in a place actually occupied by 

 hot and dry ground. 



Explanation. — An increase of temperature downwards, within 

 a few inches of the ground, at a rate considerably exceeding ^ 

 of a degree Cent, per foot. This will produce an upward bend- 

 ing of rays and a quasi reflection, as in (2); but the seeming- 

 mirror is now below the observer's eye instead of above it. The 

 flickering movements of the reflected images thus seen, due to 

 currents of hotter and colder air, greatly resemble the appear- 

 ances produced by the rippling of waves on a lake; but pro- 

 bably the most irresistible feature in the illusion is the gleam of 



