SOME NEW OPTICAL ILLUSIONS. 5 
shall observe that it is apparently shrinking together and retreating 
from us. The opposite effect — that of apparent enlargement and 
approach — is produced as a subjective compensative action after 
watching objects from which we are retreating. The effect is more 
amusing if, after observing either of these cases of motion, we 
transfer our gaze to the face of a fellow passenger sitting opposite. 
An observer at some slight elevation above a railway, seeing two 
trains pass along the lines simultaneously in opposite directions, 
will receive the impression as of one long train moving round a 
circle. For when you look at a revolving wheel nearly edgewise, 
the nearer edge is seen moving past the farther edge, and in an 
opposite direction. The apparent motion of the two trains is the 
converse of this impression. 
If from a similar situation two trains are observed, one moving 
rapidly, the other slowly in the same direction, the slower train 
may appear indeed to be moving in an opposite direction — a 
phenomenon similar to that of the Rhine above Schaff hausen 
already noticed. 
Dr. F. Guthrie has noted the following illusion : — Looking at 
the arms of a windmill in motion, in the twilight, or at such a 
distance that their attachment to the mill is obscure, we can, when 
the aspect is very oblique, easily imagine the arms to be turning 
in the opposite direction. We then fancy we are looking at the 
other side of the mill : so that if the sails are actually towards us 
in their descent, we fancy them away from us in their descent, 
which gives the notion of rotation in the opposite direction. This 
hallucination can, after a little practice, be as readily controlled by 
the will as can the introversion of a linear drawing representing a 
solid.”' 
An analogous illusion is produced by illuminating certain 
vacuum-tubes with the sparks of induced electricity discharged 
^ Guthrie, Magnetism and Electricity, p. 243. 
