MR. THOMPSON'S "ILLUSIONS." 131 



Bristol, has recently presented the French Society of Physical Science with 

 a curious example of optical illusion, the true cause of which is not clearly 

 known, but which we may compare with other facts made known some time 

 ago, of which no precise explanation has been given. Let us first consider in 

 what the effect discovered by Mr. S. P. Thompson consists, according to the 

 description that has been given of it by M. C. M. Gariel ; the illustrations 

 here given will also allow of our verifying the truth of the statements. 



The first illustration consists of a series of concentric circles of about 

 the width of a millimetre, separated by white intervals of the same size 

 (fig- l 35)- These dimensions are not absolute ; they vary with the distance, 

 and may even be a few inches in width if it is desired to show the pheno- 

 menon to a rather numerous auditory. If we hold the design in the hand, 

 and give it a twirl by a little movement of the wrist, the circle appears to 

 turn round its centre, and the rotation is in the same direction, and is equally 



Fig. 135. Fig. 136. 



Mr. Thompson's optical illusion. Give a circular movement to these figures, and the circles will appear to turn round. 



swift ; that is to say, the circle appears to accomplish a complete turn, 

 whilst the cardboard really accomplishes one in the same direction. For 

 the second effect we draw a dark circle, in the interior of which are placeid 

 a number of indentations at regular intervals (fig. 1 36). Operating in 

 the same manner as described above, this notched wheel appears to turn 

 round its centre, but this time in a different direction from the real move- 

 ment. In this, however, as in the other design, the effect is more satisfactory 

 if we do not look directly at it ; the movements also are particularly- 

 striking in combinations such as that represented in fig. 137, in which the 

 multiplicity of circles does not allow us to fix one specially. We may add 

 that the same effects may be obtained with eccentric wheels, or even with 

 other curves than circles. By means of a photograph on glass, Mr. Thompson 

 has been able to reflect these designs on a screen where they were obtained 

 on a large scale ; a circular movement was communicated to the photo- 

 graphic plate, so that the design moved in a circular manner on the screen, 

 and in this case also there existed the illusion that every circle seemed 



