400 THE BODY AT WORK 



vessel filled with ink and water, is astonished that it looks so 

 small. Nor are we prepared to accept the evidence of a 

 camera that the sun at the zenith does not produce a smaller 

 image on the retina than the sun when rising above the horizon. 

 Yet if a photographic plate is exposed to the rising sun, and 

 again, without changing its focus, to the sun at the zenith, the 

 two images are practically equal. There is a slight difference 

 due to the greater refraction of rays passing tangentially through 

 the atmosphere, but it is so slight as to bear no relation to the 

 difference between our two judgments of size. When the sun is 

 rising behind trees and houses, we compare it with objects which 

 we know to be large and distant ; yet it looks almost as large 

 when rising out of the sea. One of the causes of the illusion is 

 our conviction that the sky is flattened ; and this, again, is due 

 partly to its paler tint its less substantial blueness near the 

 horizon, and partly to our impression that it is spread out 

 over a flat earth. When the sun is in what we deem to be 

 the more distant part of the vault of heaven, we judge it to be 

 farther from us, and therefore larger than when it is above us. 

 Yet the last word has not been said in explanation of a pheno- 

 menon which has been studied by mankind since the dawn of 

 science. Helmholtz attributed the apparent greater distance, 

 and consequent greater size, of the sun and moon when near 

 the horizon to the indistinctness of their discs. When its 

 image is so reflected from the zenith as to cause the moon to 

 appear to rest upon the horizon, it does not, he said, increase 

 in size. In answer to Helmholtz's explanation, it may be 

 objected that, when at midnight he brought the full moon 

 down from the zenith, he did not bring with her the conditions 

 of light and colour by which she is customarily surrounded 

 when floating on the horizon. If, when watching the moon 

 which has just risen, vast in diameter, out of the sea, one 

 interposes between it and the eye & sheet of paper in which a 

 small hole has been made, and looks at the moon with one eye 

 through the hole, it instantly shrinks to the size which it 

 appears to have at the zenith. It is not even necessary to 

 blot out the whole of its trail of light on the sea. At the same 

 time, it appears to retreat to a great distance. This shows 

 how complicated are the associations upon which judgments 

 of size and distance are based, and to how small an extent 



