OPTICAL ILLUSIONS OF MOTION. 519 



OPTICAL ILLUSIONS OF MOTION. 



By SILVANUS P. THOMPSON, B.A., D. SC. 



THERE are frequent occasions of conflict between the receptive 

 faculties of the senses and the reflective faculties of the intellect, 

 occasions on which the mind, prejudging of the sensation received, 

 assigns it to a non-existent cause. Of all the senses none is more fre- 

 quently the seat of such deceptive judgments than that of sight ; and 

 in the science of physiological optics a very considerable share of 

 attention is claimed by optical illusions. For the purposes of conven- 

 ience, we may draw a distinction between these illusions, which are 

 the direct result of certain properties or imperfections of the eye as an 

 optical instrument, and those which arise from obliquities of judgment 

 in interpreting the sensations optically impressed upon the retina of 

 the eye. In practice, however, it is almost impossible to draw a hard- 

 and-fast line between the two classes of illusions, almost all partaking 

 of both characters. Thus, for example, it has lately been shown that 

 we habitually draw geometrical forms too large in the horizontal 

 dimension as compared with their vertical dimension ; we draw oblate 

 ellipses where we intend to draw circles ; the explanation of this being 

 that with our two eyes we really see spheres as oblate ellipses. Here 

 is, in fact, an illusion of pure association yet based upon the facts of 

 physical and physiological optics. So, again, certain inequalities in 

 the curvature of the lenses of the eye, producing the optical defect of 

 astigmatism, cause objects that are horizontal in position to form im- 

 ages at shorter (or longer as the case may be) distances from the eye 

 than the images of vertical objects ; the result being that, unless the 

 defect is corrected by suitable lenses, vertical and horizontal objects 

 (such as the bars of a window) do not appear to be at the same dis- 

 tance from the observer, though really equally remote. This would, 

 at first sight, appear to be a purely physical illusion, and not psycho- 

 logical. Nevertheless, a little consideration will show that since our 

 perception of distance is a psychological factor in the case, and that 

 this perception is based in part upon the muscular sensations of adjust- 

 ment of the lenses of the eye to exact focus, the illusion is one which 

 has a psychological as well as a physical raisoti d'etre. Again, take 

 some illusions ordinarily supposed to be one purely of mental associa- 

 tion : the common illusion of every day, that the sun or moon when a 

 few degrees from the horizon looks larger than when high in the sky, 

 appears at first sight to be due simply to the fact that when the orb is 

 near the horizon the distant objects upon that horizon whose size we 

 know, or can judge of, appear relatively small, and the sun's disk rela- 

 tively large in fact, that the illusion is one purely of association of 

 ideas. Nevertheless, when we look a little closer into the matter, we 



