WILL. 513 



tionarj surroundings tlius a^^pear in moyement, according 

 to Mach, because we are constantly innervating our eye- 

 balls to resist the drag exerted upon tliem by the pattern 

 or the flowing waves. I have myself repeated the observa- 

 tion many times above flowing streams, but have never suc- 

 ceeded in getting the full illusion as described by Macli. 

 I gain a sense of the movement of the bridge and of my 

 OAvn body, but the river never seems absolutely to stop : it 

 still moves in one direction, whilst I float away in the other. 

 But, be the illusion partial or complete, a different ex- 

 planation of it from Professor Mach's seems to me the 

 more natural one to adopt. The illusion is said to cease 

 when, our attention being fully fixed on the moving oil-cloth, 

 we perceive the latter for what it is; and to recommence, on 

 the contrary, when we perceive the oil-cloth as a vaguely 

 moving background behind an object which we directly 

 fixate and whose position with regard to our own bod}- is 

 unchanged. This, however, is the sort of consciousness 

 which we have whenever we are ourselves borne in a vehicle, 

 on horseback, or in a boat. As we and our belongings go 

 one way, the tvhole background goes the other. I should 

 rather, therefore, explain Professor Mach's illusion as 

 similar to the illusion at railroad-stations described above 

 on page 90. The other train moves, but it makes ours seem 

 to move, because, filling the window as it does, it stands for 

 the time being as the total background. So here, the 

 water or oil-cloth stands for us as background iiherhaiipt 

 whenever we seem to ourselves to be mo\-ing over it. The 

 relative motion felt hj the retina is assigned to that one of 

 its components which we look at more in itself and less as 

 a mere repoussoir. This may be the knot above the oil- 

 cloth or the bridge beneath our feet, or it may be, on the 

 other hand, the oil-cloth's pattern or the surface of the 

 swirling stream. Similar changes may be produced in the 

 apparent motion of the moon and the clouds through which 

 it shines, by similarly altering the attention. Such altera- 

 tions, however, in our conception of which part of the vis- 

 ual field is substantive object and which part background, 

 seem to have no connection with feelings of innervation. I 



