1879*] Some New Optical Illusions . 235 
smoothly and without jolting, it is extremely difficult to tell 
whether your own train or the other one is in motion. 
So when light clouds are drifted across the moon, one can 
frequently hardly resist the notion that it is the moon that 
is sailing along amongst fixed clouds ; and if the drifting of 
the clouds be due to an upper current, while the lower air 
is still, the impression that the moon is sailing along past 
the clouds asserts itself with remarkable force. 
I have observed an illusion closely akin to this at Clifton. 
Underneath the famous Suspension Bridge a zigzag path 
winds up to the top of the cliff, shaded overhead by trees. 
Walking up this path you see the bridge at intervals between 
the boughs, and, as the body rises and falls with the motion 
of each step, the bridge appears to be swaying violently up 
and down, as if it were blown about in the wind. 
Many illusions akin to these very simple phenomena have 
been recorded from time to time. Three times — in 1845, 
1848, and 1861 — the late Sir David Brewster drew the 
attention of the British Association to some phenomena seen 
in railway travelling. If from the window of the carriage 
you look out at the pebbles and stones lying beside the 
line, you catch merely vague stripes, due to the rapid motion 
of their images across the retina ; but on suddenly shutting 
the eyes “ a motion is perceived in a direction transverse to 
the real impressions on the retina ; and there is the appearance 
of lines complementary in the same transverse direction.”* 
This Sir David subsequently referred to a subjective opposite 
motion going on simultaneously, and so causing a compen- 
sation of the impressions moving on the retina. In 1861 he 
returned to the observation, and compared the phenomenon 
with that obtained by watching the motion of a rotating 
disk with radial markings, directing the eye first to a point 
near the circumference, and then afterwards to a point near 
the centre, where the motion was slower. He concluded 
that there was a neutral line across the retina at which 
the compensation of the subjective impression was com- 
plete. 
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 
Brit. Assoc. Report, 1845. 
