238 
[March 
Some New Optical Illusions . 
An analogous illusion is produced by illuminating certain 
vacuum-tubes with the sparks of induced electricity dis- 
charged 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 alter- 
nation 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 
forwards on to its bows ; for as the masts emerge from under 
the bridge, and you see them growing 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 condi- 
tion 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 reproduced. Let concentric circles 
in black and white be described upon a piece of card. 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 best 
if the circles be clear and sharp at their edges, and the suc- 
cessive rings of black and white of equal widths. Their 
number and width is immaterial, but there seems to be a 
particular distance from the eye for each width of successive 
rings, at which the illusion succeeds best. Finely-drawn 
narrow rings must be held near, to produce a maximum 
effect ; while to enable a number of persons to see the 
illusion at once the rings may be half or three-quarters of 
an inch in width, and to the number of fifteen or twenty. 
* See S. P. Thompson in Phil. Mag., 1876. 
