Scientific amusements . round. Mr. Silvanus P. Thompson, Professor of Physics atUniversity College, Bristol, has recently presented theFrench Society of Physical Science with a curious exampleof optical illusion, the true cause of which is not clearlyknown, but which we may compare with other facts madeknown some time ago, of which no precise explanationhas been given. Let us first consider in what the effectdiscovered by Mr. S. P. Thompson consists, according tothe description that has been given of it by M. C. M. Gariel;the illustrations here given will also allow of our verifyingthe truth o
Scientific amusements . round. Mr. Silvanus P. Thompson, Professor of Physics atUniversity College, Bristol, has recently presented theFrench Society of Physical Science with a curious exampleof optical illusion, the true cause of which is not clearlyknown, but which we may compare with other facts madeknown some time ago, of which no precise explanationhas been given. Let us first consider in what the effectdiscovered by Mr. S. P. Thompson consists, according tothe description that has been given of it by M. C. M. Gariel;the illustrations here given will also allow of our verifyingthe truth of the statements. 94 OPTICAL ILLUSIONS. The first illustration consists of a series of concentriccircles of about the width of a millimetre, separated bywhite intervals of the same size. These dimensions arenot absolute ; they vary with the distance, and may evenbe a few inches in width if it is desired to show thephenomenon to a rather numerous auditory. If we holdthe design in the hand, and give it a twirl by a little. Another figure of Mr. Thompsons. The different circles appear to turn roundif we give the design a rotating movement. movement of the wrist, the circle appears to turn roundits centre, and the rotation is in the same direction, andis equally swift; that is to say, the circle appears toaccomplish a complete turn, whilst the cardboard reallyaccomplishes one in the same direction. For the secondeffect we draw a dark circle, in the interior of which areplaced a number of indentations at regular in the same manner as described above, thisnotched wheel appears to turn round its centre, but this RETINA IMPRESSIONS. 95 time in a different direction from the real this, however, as in the other design, the effect ismore satisfactory if we do not look directly at it; themovements also are particularly striking in combinationssuch as that represented on page 94, in which themultiplicity of circles does not allow us to fix one ma
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Keywords: ., bookcentury1800, bookdecade189, booksubjectscientificrecreations
