1911-12.] The Fata Morgana 181 
conditions still hold, while in the other region the afternoon conditions have 
been established. 
II. The lower limit of the striated or ribbed zone of the Fata Morgana 
is continuous with the depressed line of the horizon which is associated with 
refraction in air over warm water ; the upper limit, with the elevated line of 
the horizon accompanying the refraction over cold water. 
III. The Fata Morgana shifts always from the region where the refrac- 
tions are over cold water towards that where the refractions over warm 
water still hold sway. The effect is due to the refraction over cold water 
invading the scenery point after point. 
IV. On the rare occasions on which I have observed the first appearance 
of the Fata Morgana, I have always seen it at one of the extremities of the 
circle of the horizon.* 
The general conclusions may be stated in these words : — 
(a) The Fata Morgana is made manifest at the region where the morning- 
type of refraction in air over warm water is being transformed into the after- 
noon type of refraction over cold water. 
( b ) At this region the eye of the observer placed at a convenient height 
sees simultaneously and in superposition both the depressed and the elevated 
horizons associated with the two types of refraction. 
(c) Bright objects on the lower parts of the opposite coast are stretched 
and drawn out in height between the two momentarily coexistent false 
horizons of the lake, and, by forming rectangles in juxtaposition, give the 
appearance of the banded or ribbed structure of the striated zone. In my 
memoir of 1896 I showed that the transition from the one type to the other 
does not take place slowly and progressively ; that even when towards the 
middle of the day the temperature of the air becomes equal to that of the 
water, and ere long slightly exceeds it, the depression of the apparent horizon 
and other mirage phenomena associated with the refraction over warm water 
persist for some little time. During the persistence of this mirage over the 
cold water there must be an unstable equilibrium due to the thermal 
stratification in the lower layers of air. The rapid transformation from this 
instability to the stability associated with the direct thermal gradient is the 
determining factor in the production of the Fata Morgana. The suddenness 
of its appearing and its brief transitory character are at once explained. 
* If my hypothesis is sound, the instability which leads to the Fata Morgana may occur 
at the middle of the stretch round which the phenomenon is seen as well as at the extremities. 
In such a case we should see the Fata Morgana, at first single, splitting into two moving in 
opposite directions, the one to the right, the other to the left. This possible variation in 
the details of the illusion I have searched for in vain. Should it ever be observed, it will 
be an experimentum crucis , establishing the sufficiency of my hypothesis. 
