APPARENT DISTANCE. 



78. It has been already explained that two similar objects 

 whose distances from the eye are to each other in the same 

 proportion as their linear dimensions will have the same apparent 

 magnitude. 



In like manner, if an object, such as, for example, a balloon, 

 moves from the eye in a direct line, we have no distinct conscious- 

 ness of its motion, for the line of direction in which it is seen is 

 still the same. It is true that we may infer its motion through 

 the air by the increase or diminution of its apparent magnitude ; 

 for, if we have reason to know that its real magnitude remains 

 unchanged, we ascribe almost intuitively the change of its 

 apparent magnitude to the change of its distance ; and we con- 

 sequently infer that it is in motion either towards or from us, 

 according as we perceive its apparent magnitude to be increased or 

 diminished. This information, however, as to the motion of a 

 body in a direct line to or from the centre of the eye, is not a 

 perception obtained directly from vision, but an inference of the 

 reason deduced from certain phenomena. It may, therefore, be 

 stated generally, that the eye affords no perception of direct 

 distance, and consequently none of direct motion, the term direct 

 being understood here to express a motion in a straight line to or 

 from the optical centre of the eye. 



79. The distance of a visible object is often estimated by com- 

 paring it with the apparent magnitude and apparent distance of 

 known objects which intervene between it and the eye. 



Thus, the steeple of a church whose real height is unknown 

 cannot by mere vision be estimated either as to distance or 

 magnitude, since the apparent height would be the same, provided 

 its magnitude were greater or less in proportion to its supposed 

 distance. But, if between the steeple and the eye there intervene 

 buildings, trees, or other objects, whose average magnitudes may 

 be estimated, a proximate estimate of the magnitude and distance 

 of the steeple may be obtained. 



For example, if the height of the most distant building between 

 the eye and the steeple be known, the distance of that building 

 may be estimated by its apparent magnitude, and the distance of 

 the steeple will be inferred to be greater than this. 



80. A remarkably deceptive impression, depending on this 

 principle, is deserving of mention here. When the disc of the 

 sun or moon at rising or setting nearly touches the horizon, it 

 appears of enormous magnitude compared with its apparent size 

 when high in the firmament. Now, if the visual angle which it 

 subtends be actually measured in this case, it will be found to be 

 of the same magnitude. How, then, it may be asked, does it 

 happen that the apparent magnitude of the sun at setting and at 



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