THE PERCEPTION OF THINGS. 91 



in which we sit, all stationary objects visible through the- 

 window give us a sensation of gliding in the opposite 

 direction. Hence, whenever we get this sensation, of a. 

 window with all objects visible through it moving in one 

 direction, we react upon it in our customary way, and per- 

 ceive a stationary field of view, over which the window, and 

 we ourselves inside of it, are passing by a motion of our 

 own. Consequently when another train comes alongside 

 of ours in a station, and fills the entire window, and, after 

 standing still awhile, begins to glide away, we judge that it. 

 is our train which is moving, and that the other train is still.. 

 If, however, we catch a glimpse of any part of the station 

 through the windows, or between the cars, of the other train, 

 the illusion of our own movement instantly disappears, and 

 we perceive the other train to be the one in motion. This^ 

 again, is but making the usual and probable inference from 

 our sensation.* 



Another illusion due to movement is explained by Helm- 

 holtz. Most wayside objects, houses, trees, etc., look small 

 when seen out of the windows of a swift train. This is be- 

 cause we perceive them in the first instance unduly near. 

 And we perceive them unduly near because of their extra- 

 ordinarily rapid parallactic flight backwards. When we 

 ourselves move forward all objects glide backwards, as 

 aforesaid ; but the nearer they are, the more rapid is this 

 apparent translocation. Relative rapidity of passage back- 

 wards is thus so familiarly associated with nearness that 

 when we feel it we perceive nearness. But with a given 

 size of retinal image the nearer an object is, the smaller do 

 we judge its actual size to be. Hence in the train, the 

 faster we go, the nearer do the trees and houses seem, and 

 the nearer they seem, the smaller do they look.f 



Other illusions are due to the feeling of convergence being 

 wrongly interpreted. When we converge our eyeballs we 

 perceive an approximation of whatever thing we may be 

 looking at. Whatever things do approach whilst we look 



* "We never, so far as I know, get the converse illusion at a railroad sta- 

 tion and believe the other train to move when it is still, 

 t Helmholtz : Physiol. Optik, 365. 



