SECT. 3] APPLICATIONS OF THE OYTtOPENDTTXTTM 343 



be exquisitely sharp (Fig. 15). Slow changes in the mean atmospheric refractive 

 index along the line of sight cause the horizon line to rise and fall each 5 min 

 or so through an amplitude of some 3 to 5 seconds of arc. An average position 

 can probably be obtained to an accuracy approaching one second of arc from 

 100 observations scattered through a total of some 20 min time. 



Under less favorable conditions, as when the "seeing" is bad, the image of 

 the horizon appears to heave violently under even moderate optical magnifica- 

 tion (Fig. 16). Mirage and loom effects also contribute important disturbances 



■ 



Fig. 16. Horizon at a range of 15 nautical miles under conditions of poor seeing. Reticle 

 interval is 60 seconds of arc per division; sea state is force 2. 



amounting to several minutes of arc. One of the most interesting of these 

 refractive effects is the pronounced vertical extension of objects on the horizon 

 when cold air moves out over warm water in the fall of the year. Not only is the 

 horizon closer but the amplitudes of low waves are greatly exaggerated. This 

 effect can be so extreme as to lead to alarming overestimates of the sea state 

 or, indeed, for wave crests to appear square shouldered or even as lenticles 

 hanging in the air. In any of these conditions the zenith angle of the horizon 

 cannot be measured with any certainty, except possibly from stations having a 

 height of eye in excess of 10 m. From such elevations the zone where light 

 passes through the steepest lapse rate of air temperature near the sea surface 

 is far enough removed from the observer to make its angular disturbances 

 small. While this provides an advantage, too much height of eye may permit 

 the haze on long light paths to make the horizon quite indefinite. 



From preliminary inspection of the image quality under a variety of tem- 

 perature conditions, it would seem that observations having sufficient coherence 



