1908] Meteorological Optics. 137 



or dome, or hemisphere. However, if we sweep the sky with the 

 ■eye from the horizon to the zenith, or the reverse, it will be seen 

 that the dome is not spherical but that it is flattened, the ap- 

 pearance being that it is farther to the horizon than to the 

 point overhead. The preceding is true whether looked at by 

 day or by night, particularly in a cloudless sky. This is easily 

 demonstrated by estimating say the point of the heavens midway 

 between the horizon and the zenith or the point overhead, and 

 then measure with an instrument the elevation of the point of 

 b)isection. It will be found that the halving point is only about 

 half as high as it appears to be. The physiological effect of 

 passing the eye from its normal position towards the horizon, to 

 overhead, is to give the impression of a depressed vault or dome, 

 and the arc we bisect is not that of a semi-circle but the segment 

 of a larger circle. Any one who has been in our Rocky Mountains 

 will recall the impression of "the giants towering to the skies," 

 l)ut when we measure their angular elevation we find the "tower- 

 ing" very much lessened; physiological effect, due to our con- 

 stitution. A similar illusion we mav notice in the apparent size 

 of constellations near the horizon. 



The most familiar object for this illusion is of course the 

 moon, although the stm shares it equally, but I suppose the 

 most of us see the moon rise more frequently than the sun, 

 reminding one of the man who when asked, if he ever saw the 

 sun rise, answered, "I don't go to bed as late as that." 



Many observations and measurements have been made on 

 the sun and moon by setting up a circular disk and viewing 

 alternately, say the moon and disk, always moving to or from 

 the disk until it appeared the same size as the moon, and then 

 measuring the distance to the disk. From such and mathematical 

 considerations it is found that the moon appears of its proper 

 size when elevated between 30° and 35°, while when it is on the 

 horizon it is nearly two and half times larger, and when high up 

 in the sky only about half as large as it should be. 



When Coleridge lets the "Ancient Mariner" say: — 



"All in a hot and copper sky, 

 The bloody Sun at noon, 

 Right up above the mast did stand, 

 No bigger than the Moon," 



he gave expression to the fact, just stated above, that our dis- 

 penser of life and light, and our satellite appear small when they 

 are high in the heavens. 



Now for another phenomenon, that we observed later as 

 the gloaming was receding. Let us paraphrase the well-known 

 couplet into, 



