SOME NEW OPTICAL ILLUSIONS. 
• 2 
When the train in which you are seated is drawn up beside 
another train^ and then moves slowly forward^ smoothly and with- 
out joltings 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 fre- 
quently 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 4 n 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 strips, 
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 com- 
pensation 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 
1 Brit. Asso. Repo^'t, 1845. 
