586 THE NERVOUS SYSTEM. 



. 



The New International Encyclopaedia regards the apparent 

 enlargement as due to an illusion of distance. The distance be- 

 tween the observing eye and the horizon seems to be longer than 

 the distance between the eye and the zenith, owing to the haziness 

 of the air, the number of intervening objects, etc. The retinal 

 image being practically the same in both instances, the object which 

 the mind refers to a greater distance appears to be larger. 



Prof. Pickering, of Harvard College Observatory, in his book, 

 The Moon, in treating of this subject, says : 



" The true angular size of the moon is about half a degree ; it 

 can, therefore, always be concealed behind a lead-pencil held at 

 arm's length. The sun and moon when rising or setting appear 

 to most persons of from two to three times the diameter that they 

 have when near the meridian. The cause of this phenomenon has 

 been a source of speculation from the earliest times. Before optical 

 science was thoroughly developed some thought that the image 

 was really magnified by the vapors near the horizon. Not only is 

 this incorrect, but in point of fact the sun is slightly and the moon 

 measurably smaller when near the horizon, because they are farther 

 off than when overhead. 



" The true explanation is twofold. Human estimates of angular 

 dimensions are dependent not merely on the angular dimensions 

 themselves, but also on several extraneous circumstances. The 

 case is analogous to our estimates of height, which are dependent 

 primarily on the real height of the object, but secondarily upon 

 its bulk. Thus, a pound of lead feels much heavier than a pound 

 of feathers. 



" One circumstance affecting our estimates of angular dimension 

 is the linear dimension of the object itself. It was pointed out by 

 Alhazen about nine hundred years ago that if we hold the hand at 

 arm's length and notice what space it apparently covers on a dis- 

 tant wall, and then move the hand well to one side so that it is in 

 front of some very near object, we shall find that it will appear to 

 us decidedly smaller than the part of the wall which it previously 

 covered. ^ It is an analogous effect which makes the full moon 

 when rising or setting appear larger than when it is well up in the 

 sky. On the horizon we can compare it with trees and houses, 

 and see how large it really is ; overhead we have no linear scale 

 of comparison. 



" It is certain that this is not the only reason, however, nor 

 even the chief one, that makes the moon appear larger when near 

 the horizon. The same optical illusion appears when at sea, and 

 it applies also to the constellations for example, to Orion. When 

 rising they appear decidedly larger than when near the meridian, 

 and yet no comparison of their size with that of terrestrial objects 

 is usually possible. There is evidently another circumstance 



