236 Some New Optical Illusions . [March, 
muscles of the eye-ball, continuing after the gaze had 
been directed to the rocks, and thus occasioning a displace- 
ment of the images on the retina.* 
This illusion becomes more remarkable in the slightly 
varying case now to be mentioned. Watch the water of a 
rapid river, such as the Rhine immediately above Schaff- 
hausen. The middle stream is running forward very rapidl} 7 . 
After watching it fixedly for some time, transfer your gaze 
to the slower stream near either bank. It actually seems 
to be running back. 
I have also noticed, after watching a procession, that 
stationary objects appeared for a moment to be moving in a 
contrary direction. 
In the “ Journal of the Royal Institution ” (vol. i., p. 609) 
an anonymous writer records a curious observation, that 
from a slowly-moving railway-train, while the stones and 
sleepers beside the line appear to fly back past the train, the 
neighbouring set of rails seems to be flying forward and 
keeping pace with the train. This he refers, and doubtless 
rightly, to the fadt that the rails are of nearly uniform tint, 
and destitute of markings that would produce upon the 
retina impressions like those of the adjacent objedts. 
The railway affords many other instances of optical de- 
ception, and of these I will mention a few of which I am 
not aware that any specific notice has hitherto been taken. 
When a landscape is observed from a moving railway- 
train, all distant objedts from the near hedgerows to the 
distant hills appear to be moving past in the opposite direc- 
tion, the nearer objedts having the greater apparent velocity. 
Consequently, if the attention be fixed upon any objedt at 
some distance from the line, all objedts beyond will relatively 
appear to be moving forward with the train, while objedts 
nearer appear to be moving backwards. The combined 
effedt is to make the entire landscape appear to be revolving 
centrally round whatever point we fix our attention upon. 
Falling rain seen from a moving train always seems to 
fall obliquely (except in a very strong gale in the diredtion 
of the train’s motion) in a diredtion opposite to that of the 
motion of the train. But if another train happen to pass 
in an opposite diredtion, and we look out at this and follow 
it with our eyes, rain-drops falling between the two trains 
will seem to be flying forward with ourselves. 
* An account of a very similar observation was communicated by Mr. J. 
Aitken to the Royal Society of Edinburgh, in November, 1878, apparently 
without any knowledge of the observations of Addams, Brewster, or cf the 
author of this article. 
