ESTIMATE OF REAL MOTION. 



On entering, for example, the aisle of St. Peter's at Rome, or 

 St. Paul's at London, we are not immediately conscious of the 

 vastness of the scale of these structures ; but if we happen to see 

 at a distant part of the building a human figure, we immediately 

 become conscious of the scale of the structure, for the known 

 dimensions of this figure supply a modulus which the mind 

 instantly applies to measure the dimensions of the whole. For 

 this reason artists, when they represent these structures, never 

 fail to introduce human figures in or near them. 



83. It has been explained that the apparent magnitude of 

 objects depends conjointly on their real magnitude and their 

 distance. Although, therefore, the eye does not afford any direct 

 perception t either of real magnitude or distance, we are by habit 

 enabled to infer one of these from the other. 



Thus, if we happen to know the real magnitude of a visible 

 object, we form an estimate of its distance from its apparent 

 magnitude ; and, on the other hand, if we happen to know or can 

 ascertain the distance of an object, we immediately form some 

 estimate of its real magnitude. 



Thus, for example, the height of a human figure being known, 

 if we observe its apparent visual magnitude to be extremely small, 

 we know that it must be at a distance proportionally great. If 

 we know that at 20 feet the figure of a man will have a certain 

 apparent height, and that we find that his figure seen at a certain 

 distance appears to have only one-fifth of this height, we infer that 

 his distance must be about 100 feet. 



In like manner, the real magnitude may be inferred from the 

 apparent magnitude, provided the distance be known or can be 

 ascertained. Thus, for example, in entering Switzerland by its 

 northern frontier, we see in the distance, bounding the horizon, 

 the line of the snowy Alps, and the first impression is that of 

 disappointment, their apparent scale being greatly less than we 

 expected ; but when we are informed that their distance is sixty 

 or eighty miles, our estimate is instantly corrected, and we 

 become conscious that the real height of mountains which, seen 

 at so great a distance, is what we observe it, must be pro- 

 portionately vast. 



84. When an object moves in any direction which is not in a 

 straight line drawn to or from the centre of the eye, the direction 

 in which it is seen continually changes, and the eye in this case 

 supplies an immediate perception of its motion ; but this perception 

 can be easily shown to be one not entirely corresponding to the 

 actual motion of the object, but merely to the continual change of 

 direction which this motion produces in the line drawn from the 

 object to the eye. 



89 



