May 4, i8S8.] 



SCIENTIFIC NEWS. 



417 



OPTICAL ILLUSIONS. 



TEN years ago Professor Silvanus P. Thompson, now 

 the Director of the City and Guilds of Finsbury 

 College, described some designs which gave rise to 

 optical illusions, and which had not previously been 



Fig. I. 



observed. These figures he called strobic circles ; they 

 consisted of concentric circles which appeared to revolve 

 when the drawing was rotated eccentrically by a hori- 

 zontal movement similar to that employed in rinsing a 

 basin. The circles in figure i, especially if drawn on a 



Fig. 



much larger scale, show this phenomenon very plainly. 

 The figure need only be placed in the palm of the hand 

 and moved rapidly with a circular motion, when the 

 circles will appear to revolve. 



Recently some new and curious illusions of an 

 analogous nature have been communicated to La Nature, 

 from whose pages we borrow our account. The rosettes 

 in figure 2 appear to turn when a rotary motion is given 



to the design, and as the alternate black and white 

 stripes are more distinct than in figure i the pheno- 

 menon is easily seen, even on the scale of our illus- 

 tration. 



Figures 3 and 4 undergo a very curious change when 

 they are rotated on their centres like ordinary wheels. 



Fig. 2. 



The radial stripes in figure 3 disappear, and instead there 

 appears a circle of which the circumference is limited by 

 the centres of the actual circles. Outside this circum- 

 ference, if the movement be not too rapid, numerous 

 semicircles are seen. 



Fig. 4. 



On rotating figure 4 on its centre its appearance is 

 completely changed, and the parallel lines are apparently 

 converted into concentric circles. 



The figures are one quarter the size of the discs ex- 

 perimented with, and our readers will find the effects 

 much more striking if the figures be drawn to the full 

 size. For the discs 3 and 4 it is only necessary to turn 

 them by hand on a pin, or on the point of a pencil. 



