FALLACIES OF VISIOX. 



impressions are fallacious. Let any one adopt any convenient 

 method which may occur to him, to measure the apparent mag- 

 nitude of the sun on the horizon, and again on the meridian, and 

 he will find them the same. This may be accomplished by 

 extending two threads of tine silk parallel to each other in a 

 frame, and placing them in such a position, and at such a distance 

 from the eye, that when presented to the sun or moon, on the 

 horizon, they will, exactly, touch its upper and lower limb, so 

 that their apparent distance asunder will be equal to the 

 apparent diameter of the lunar or solar disk. If this arrange- 

 ment be preserved, and the sun or moon be viewed in the same 

 manner when at, or near, the meridian, it will be found that the 

 threads will equally touch its upper and lower limbs, and that 

 their interval will still measure its apparent diameter. It 

 will, therefore, be evident that whatever be the cause of the 

 illusion, the apparent magnitude of the sun or moon is not 

 greater at rising or setting than in the meridian. Whence, 

 then, it may be asked, arises an impression so universally enter- 

 tained 1 



The explanation of this singular effect, in which all astronomers 

 appear to concur, refers it to mental, and not optical causes ; 

 strictly speaking, it is not an optical illusion. The error is 

 one of the mind and not one of the senses. The estimate which 

 we form of the actual magnitude of any visible object depends 

 on a comparison of the apparent magnitude which that object 

 presents to the eye, with the distance at which ~we imagine 

 it to be. Thus if there be two objects buildings, for example, 

 which have to the eye the same apparent height, but which we 

 know or believe to be at different distances from us, we instinc- 

 tively, and without any operation of the judgment of which 

 we are conscious, conceive that which is more distant to be the 

 largest. 



To apply this reasoning to the case of the sun or moon, we 

 are to consider that when either of these objects is in the 

 horizon, a portion, at least, of the space between the eye and 

 it is occupied by a series of objects with the magnitudes and 

 relative positions of which we are familiar. We are, therefore 

 enabled to make some estimate of a portion of the space that 

 intervenes between the eye and the object. But when the 

 object is in a more elevated position in the firmament, no part of 

 the intervening distance is thus spaced out, and we are accus- 

 tomed to consider the object nearer to the eye. 



Conceding this, then, it will be asked how it explains the 

 universal impression of the enormously large disk of the sun or 

 moon when rising or setting ; the answer is, that when in or 



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