SOME NEW OPTICAL ILLUSIONS. 
3 
radial markings, directing the eye first to a point near the circum- 
ference, and then afterwards to a point near the centre, where 
the motion w^as slower. He concluded that there was a neutral 
line across the retina at which the compensation of the subjective 
impression was complete. 
In the ‘‘Philosophical Magazine ” for 1834 (p. 373) R. Addams 
described a peculiar optical phenomenon. After looking for some 
time at a waterfall, and then at “the sombre water-worn rocks 
immediately contiguous,’^ he “ saw the rocky surface as if in 
motion upwards with an apparent velocity equal to that of the 
descending water.” This he ascribed to an unconscious recurrent 
movement of the 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 rapidly. 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 fact that the rails are 
^ An account of a very similar observation was communicated by Mr. J. 
Aitkin to the Royal Society of Edinburgh, in November, 1878, apparently 
without any knowledge of the observations of Addams, Brewster, or of the 
author of this article. 
