6 
SOME NEW OPTICAL ILLUSIONS. 
alternately in opposite directions^^ when the tube appears to be 
rotating about an axis perpendicular to its length and to the line 
of vision. 
A crow flying along at dusk^ seen against the sky at a low 
altitude, shows, when passing the observer, his wing above and 
beneath his body alternately, The effect of this alternation is as 
if he had but one wing, which seems to revolve round like the 
blade of a screw-propeller about its axis. 
I have frequently stood upon the lofty suspension-bridge over 
the Avon, at Clifton, when large ships have been passing beneath. 
Under these conditions a curious illusion may be observed. If 
you look perpendicularly down on to a ship, as it emerges from 
beneath, it appears to be heeling forward on to its bows ; for as 
the masts emerge from under the bridge, and you see them grow- 
ing longer as the fore-shortening effect passes off, the mind cannot 
resist the notion that — like the windmill-sails— they are revolving 
round a centre. I have pointed out this effect to several persons, 
who have expressed much surprise at the completeness of the 
illusion. 
The last set of illusions which will be described took their 
origin in an observation made by the writer early in 1876. He 
had been drawing a series of concentric circles in black and white, 
for the purpose of testing the astigmatic conditions of the eye. 
Happening to shake the paper upon which the diagram was 
drawn, he observed a peculiar motion of apparent rotation of the 
circles. This illusion is extremely curious, and very easily repro- 
duced. Let concentric circles in black and white be described upon 
a piece of card f FL In fig- i)- If this be held firmly between the 
thumb and finger of the hand, and then a slight but rapid circular 
shaking motion be imparted by the wrist and elbow, the circles 
will appear to rotate upon the card. The hallucination succeeds 
1 See S, P. Thompson in Phil. Mag., 1876. 
