i879*J 
237 
Some New Optical Illusions . 
If we stand on the platform of a station and watch a 
train approach, the end of the engine appears to enlarge or 
swell up as it approaches and occupies a larger area of the 
field of vision. Conversely the end of the last carriage of 
a retreating train appears to shrink down and contra<5t as it 
diminishes in apparent magnitude. Stationary objects by 
the side of the line similarly appear to swell up as we ap- 
proach them in a train, and to shrink together as we retreat 
from them. Curiously enough, this motion is also one which 
calls forth a certain “ compensation ” in the adtion of the 
retina. For, suppose we have been watching objedts en- 
larging as we approached them, and then suddenly transfer 
our gaze to the side of the carriage opposite to us, we shall 
observe that it is apparently shrinking together and retreating 
from us. The opposite effedt — that of apparent enlargement 
and approach — is produced as a subjedtive compensative 
adtion after watching objedts from which we are retreating. 
The effedt 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 op- 
posite diredtions, 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 diredtion. 
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 diredtion, the 
slower train may appear indeed to be moving in an opposite 
diredtion — a phenomenon similar to that of the Rhine above 
Schaffhausen already noticed. 
Dr. F. Guthrie has noted the following illusion : — “ Look- 
ing 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 aspedt is very oblique, easily 
imagine the arms to be turning in the opposite diredtion. 
We then fancy we are looking at the other side of the mill : 
so that if the sails are adtually towards us in their descent, 
we fancy them away from us in their descent, which gives 
the notion of rotation in the opposite diredtion. This hal- 
lucination can, after a little pradtice, be as readily controlled 
by the will as can the introversion of a linear drawing repre- 
senting a solid.” * 
* Guthrie, Magnetism and Eleftricitj’, p. 243. 
