356 Proceedings of Boy al Society of Edinburgh. [sess, 
rasch iind willkiirlicli mit den beiden Eindriicken wechseln. Jedes Verfolgeii 
des Teppichs bringt den Beobacbter zum Steben, jedes Fixiren von K oder 
Nichtbeachten des Teppiclis, wobei dessen Zeicbnungen verschwimmen, setzt 
den Beobacbter in Bewegung.” (Pp. 65, 66.) ^ 
My experience does not coincide with that of Prof. Mach, as 
given in the passage last quoted. When I look over the 
parapet of a bridge at the water flowing below, I see the water 
move and the bridge steady when my lines of glance are fixed 
relatively to the bridge, but I can fix points in the moving water, 
masses of froth, or eddies, or hits of stick ; and when I do so, and 
jerk my eyes from one portion of the moving mass to another, then 
my lines of glance being fixed relatively to the water, I see the 
water at rest and the bridge moving. In the same way, on a moon- 
light night, when there are light clouds in the sky, we can see the 
moon moving and the clouds fixed, or the clouds moving and the 
moon fixed, according as our lines of glance are fixed relatively to 
the clouds or to the moon. So when, in a railway station, there 
is another train alongside of ours, and one of the trains begins to 
move, we can see either our own train or the other one moving, 
that train seeming fixed which is fixed relatively to our lines of 
glance. 
