X VISUAL JUU<;MENTS 447 



tinctness with which the object is setsn leads to the wrong 

 conchision that it is situated at some considerable distance 

 from the observer. Hence the judgment is formed that 

 the object is large, because if it were not large it could 

 not, at the apparently greater distance, produce an miage 

 on the retina of the size it does. 



The moon, or the sun, when near the horizon appears 

 very much larger than when it is high in the sky. This 

 is usually said to be due to the fact that when in the 

 latter position we have nothing to compare it with, and 

 the small extent of the retina which its image occupies 

 suggests small absolute size. But as it sets, we see it 

 passing behind great trees and buildings which we know 

 to be very large and very distant, and yet it occupies a 

 larger space on the retina than they do. Hence the 

 vague suggestion of its larger size. But this has really 

 very little to do with the delusion, for the appearance is 

 the same if the sun or moon is seen near the horizon 

 over the open sea, where no comparison with other 

 objects is possible. Probably one cause of the delusion 

 is that when low down the sun or moon is seen less dis- 

 tinctly, on account of mist. and vapour, and thus ''looks" 

 large for the same reason that a man seen in a fog appears 

 unduly big, or the delusion may be due to the fact that 

 to most people the distance from them to the horizon 

 appears greater than the distance straight above them 

 to the summit of the vault of the heavens (or the zenith). 

 Hence though the actual size of the image of the sun 

 or moon on the retina is the same whether they be low 

 down or high up, the idea that they are further off when 

 low down suggests that they are of greater size. 



8. Judgment of Form by Shadows.— If a convex 

 surface be lighted from one side, the side towards the 

 light is bright — that turned from the light, dark, or in 

 shadow ; while a concavity is shaded on the side towards 

 the light, bright on the opposite side. 



If a new half-crown, or a medal with a well-raised head 



