SOME MENTAL ILLUSIONS OE VISION. 



K. T. Li:\VIS. l-.K.M.S. 



Notwithstanding the popular saying that "seeing is 

 believing," neither of our faculties is subject to such complete 

 illusions as our sense of sight, some being optical, others 

 ocular, whilst again others are purely mental. 



To the first kind belong those due to refraction — such for 

 instance as the mirage, or the apparent displacement of an 

 object from its true position ; to the second kind may be 

 referred the phenomena of irradiation, and of subjective 

 images and colours ; whilst the third includes the impression 

 which most persons have of the apparent increase 

 of the San and Moon when rising or setting, and 

 the similar increase in the apparent size of con- 

 stellations, or the distance apart of stars when 

 seen near the horizon, as compared with our 

 estimate of such at their meridian altitude. 



That this last-named illusion is neither optical 

 nor ocular is shown at once by the fact that 

 when seen through a telescope no such apparent 

 increase is found to exist ; indeed, when accur- 

 ately measured, after making all necessary correc- 

 tions for height of barometer, temperature, and 

 refraction in altitude, it is found that the appar- 

 ent diameter of either body is really greatest when 

 on the meridian, inasmuch as they are then some 

 4,000 miles nearer to us than when near the 

 horizim. though the difference is too small to be 

 appreciated by the naked eye. 



It should be noted, however, that the amount of 

 the apparent augmentation varies considerably 

 with individuals — artists for instance depict the 

 rising moon of a variety of e.xaggerated sizes up 

 to about 15° diameter, or about thirty times 

 greater than it should be drawn, and one not 

 infrequently hears it described as looking " as 

 large as a dinner plate." whereas a three-penny 

 bit held at arm"s length is rather larger than is 

 necessary to hide it completely. 



That this almost universal illusion is entirely mental, is not 

 easy to demonstrate, although it will be readily admitted that if 

 a false impression of the distance of an object can be created, 

 the mental estimate of its actual size will be proportionately 

 altered : in other words, if an object we believe to be near to us 

 appears of a certain size, and one of the same kind believed 

 to be at a distance appears also of the same size, we at once 

 from experience correctly judge that the latter must be the 

 larger of the two. Hence though the Moon, whether high or 

 low in the sky, practically subtends the same angle, and 

 therefore its image on the retina of the eye is in both positions 

 the same, we naturally from experience imagine it nmst be a 

 larger body in the latter case, because though so much further 

 off it appears to be of the same size as the Moon we 

 see when apparently nearer. 



The experiment referred to in the accompanying diagram 

 affords a convincing illustration of the curious mental eflect 

 produced by the creation of a false impression of distance. 

 and is one which may be performed by any persons who 

 possess sufficient control of the eyes to dissociate the focussing 

 and the convergence, which, though (|uite separate and 

 independent actions, are from constant habit usually performed 

 automatically together. .\-B represents a stereoscopic slide, 

 which we will suppose to be of the full moon, placed at the normal 

 distance of 10 inches from the eyes, when the two pictures 

 are seen clearly in focus. If whilst retaining the focal distance 

 we alter the convergence so that the axis of the right eye (R) 

 passes centrally through the picture B, and that of the left 



eye (D through the centre of A, as shown by the two firm 

 lines L-C and K-C, these axes will meet at the point C, which 

 will be more or less distant from the plane of .A.-B according 

 to the width apart of the eyes of the observer. The result of 

 this will be that three images of the pictures A and B will be 

 seen, of which the central one, consisting of A and B super- 

 posed, will be seen apparently at C, and of course perfectly 

 stereoscopic. If then without altering the distance between the 

 eyes and the slide we look so " cross eyed '" that the axis of K 

 passes centrally through A. and that of L through B, as indicated 

 by the two dotted lines, we shall again see three 

 images, the centre one apparently at D. but in 

 this case pseudoscopic. It is clear that in both 

 cases the images of the pictures on the sympa- 

 thetic portions of the retina will be approximately 

 the same, but in the former case where the 

 moon is seen apparently at C, or nmch further 

 off than it really is, the effect is that of a consider- 

 ably magnified image, whereas in the latter case 

 where the picture is seen apparently at D, or much 

 nearer than it really is, the mental impression 

 produced (after making due allowance for the 

 difference between R-A and R-B) is that of a 

 considerably smaller moon than either of those 

 seen at A-B, the mental illusion in each case 

 being due to the fact that a false idea has been 

 created as to the distances of the objects observed. 

 .A further experiment may be suggested by 

 means of the subjective image of any object which 

 gazed upon long enough to produce a con- 

 tinued impression upon the eyes. If, for instance, 

 the filament of an incandescent electric lamp is 

 steadilv looked at for a few seconds, the impres- 

 sion produced by it upon the sympathetic por- 

 tions of the eyes will persist until the vibrations 

 so excited have run down, and a spectral image 

 of the filament will meanwhile be seen so long 

 as the convergence of the eyes is maintained at the same 

 angle as when the lamp was originally looked at. If this 

 image is projected upon the distant wall of the room it will 

 appear of magnified proportions, whereas if thrown upon some 

 surface nearer to the eye than the exciting cause, it will 

 apparently be seen of very reduced dimensions, although in 

 both cases the actual picture formed must be preci-sely 

 the same. 



As yet another, though somewhat difterent class of mental 

 illusion in the case of vision, I may mention that I have before 

 me a photograph of the "' Challenger " medal, taken under a 

 strong oblique light, falling upon it from the top. Knowing 

 from experience that if the light falls in that direction upon a 

 raised surface, the shadows would fall as depicted, the mental 

 impression produced when the top of the photograph is held 

 towards a source of light is that of a design in strong relief, 

 although the sense of touch assures me it is flat : but if the 

 picture is turned round so that the light falls upon it from the 

 bottom, experience immediately suggests that shadows, as 

 they then appear, can only be thrown by an intaglio, and the 

 mental suggestion is that of a sunk pattern, an illusion 

 which would be complete but for the fact that the inscription 

 still reads the right way about. 



A false estimate of distance, and therefore of size, is very 

 conuuon to persons in the clear air of high mountainous 

 regions, who under ordinary circumstances are accustomed 

 to see landscapes through our own more murky atmosphere, 

 and the common illusion of meeting suddenly with gigantic 



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